For the most part, your garden variety of plants wouldn't be affected too much by the force gradient. There actually has been quite a bit of research put into this. I'm trying to find a source for this, but a few years back the Japanese burned a bunch money on this kind of research and didn't end up with anything substantial. If you do it on a smaller scale, just for the plants, it also creates the issue of extra angular momentum added to your spacecraft. This is manageable, but definitely something to take into account.
But, you're right. If you created gravity on a scale large enough for humans, it would definitely get rid of most of the problems for the plants.
In the context of this thread, I'm mostly interested because it seems like a happy middle ground to solve your capillary action issues. However it would also be super interesting to see how a small acceleration like that could help with e.g. retinal problems from microgravity.
With NASA's current budget, they probably figure it's easier to just deal with microgravity for a spaceflight with a limited duration (like to Mars). Generating gravity only really makes sense for spaceflights of longer/unlimited duration (interstellar travel).
A capsule tethered to a second stage, or a pair of capsules is the low-cost* way to do it. The technique was demonstrated during the Gemini missions (at very low g).
*Compared to rotating space station
As to why... I suspect non-ISS life science research in orbit just has no where to get funding or support.
Growing crops in space has no real scientific benefits for the nearterm future. It's a "nice to have" not a "visionary" thing.
The science resulting from our robot exploration is astounding. Nearly all photos of outer space objects beyond the earth and moon are robot driven, and the high quality photos of Jupiter, etc are entirely robot driven. Compare that to a Mars rock.
BTW, we can't send a human to Mars right now. It's technically impossible- well, I mean, we could deliver their dead body to the surface, but that's not super useful.
A single geologist on Mars would constantly have to check back in on their plans.
Mars is not. It's a minimum of a 1-hour round-trip. So every tiny little rover movement has to be planned in advance, unless you can figure out how to automate it, which only goes so far: you're not going to put an AI with a geologist's education in a rover.
This is not likely to ever change, unless you can 1) figure out how to make FTL communications work (subspace?), or 2) figure out how to make AIs with human-level intelligence (at which point we humans would be completely obsolete and likely slated for extermination). Good luck with either of those. Until then, sending actual humans is the only way to get a lot done in a reasonable amount of time.
The nice photos you talk about from robotic exploration is the result of completely pre-planned missions, with very little possibility of needing to make fast decisions on-site. When you're just flying around the Jupiter system or flying through the Pluto system, that's really not that hard; the planet and moons' locations can be predicted with a very high degree of accuracy, and the data they're gathering is all planned far in advance. It's not like walking around a planet's surface and deciding which rock to investigate, or which rock to ignore, or what a good place to dig would be. You can do all that by remote control, but it's a whole lot slower because you need to have humans verify everything, so what would take you an hour with a human on-site now takes you days.
The beauty of large-scale robotic exploration is that you can use all the analysts on earth to analyze the results. Compared to a single human.
Yes, NASA has made mission changes post-launch, but these are usually simple changes, consisting basically of single orders to redirect the craft along a new trajectory. A human walking around looking at rocks makes those decisions on a second-by-second basis.
Particulates, like soil, are actually considered a hazardous material in space because it can float into and clog up the ventilation.
Capillary action is a dominant force in microgravity and water will coat everything.
Roots are respiratory and will absorb the oxygen in the water around them, but the bubbles of CO2 they create won't naturally rise to the surface. This can cause the roots to suffocate.
Hydroponics are the logical route because regular soil will deplete its nutrient supply, but you still need a root growth substrate. The current "Veggie" system uses vermiculite clay chips contained in a pouch. They had to use the right gauge of particle diameter though: too small and it will saturate with water and suffocate the roots, too big and there won't be enough pressure gradient from the capillary action and the roots will dry out. Also, if you don't have your hydroponic nutrient solution just right, salts will build up in your substrate, so the pouches are typically single use.
Our team developed a method for 3D printing a growth substrate. This eliminates the problem with particulates and it also allows you control the gauge of the holes in the material to get the right air/water ratio. You can even vary the hole sizes so the water is pulled by the capillary action more at the dry end and fills the medium more evenly with water. It's not a perfect solution. The hydrophilic nylon filament we used liked to warp when it was printed. It also doesn't work for root vegetables like potatoes and stuff. However, it is reusable, cleanable, recyclable, scalable, largely passive, and safe. Plus, everyone loves 3D printers.
TL;DR Microgravity creates a lot of problems for growing plants in space, many of which aren't immediately obvious. 3D printed growth mediums might be a solution.