To the second question: since each mission is expensive and time-consuming, scientists reasonably want the best sensors for each mission.

That's both the tech upgrade during the n-year coast phase, and the investigation upgrade as they work out new things to investigate.

It's not as straightforward as spamming the system with cameras. That's bound to happen more, after the MarCO cubesats had success flying along with Insight.

> Why arent we just mass launching probes to all the planets?

Because we don't live in a fictional sci-fi universe where launching things into space doesn't cost a ridiculous amount of money, time and effort, and can't only be done at certain times from certain locations in order to get the orbits right, or where years of effort and millions of dollars (or whatever currency) don't also go into building the things to be launched into space. The world has better things to spend its money on.

>Why is it not OK to have a standard sensor array on every planet in the solar system?

What is a "standard sensor array?" We've already sent a lot of probes to a lot of places[0] around the solar system. But read the rest of this thread - it's a lot more complicated than it seems. Also, remember how hostile and difficult the solar system can be. Jovian planets don't even have a solid surface to be "on," for instance, and Venus has a pressure of 90 atmospheres and it rains sulfuric acid.

[0]https://en.wikipedia.org/wiki/List_of_landings_on_extraterre...