Or: https://en.m.wikipedia.org/wiki/Freight_bicycle

Or a bike trailer[1]. There's one for every job. Cheaper [than a low volume bike] and you get to keep/have a regular bike for when you're not carrying freight.

[1]https://www.treehugger.com/bikes/bicycle-cargo-chapter-2-bik...

Um, what? Bikes are literally the most effecient form of cargo transportation. Get a rack and a bag. Or a milk crate. It's not impractical at all.

I carry a couple days worth of groceries home regularly -- on a skateboard of all things. I can even get eggs back home without breaking them. And there's also a laptop in my bag since I'm picking these up on the way back from work.

You can't do a week's worth of shopping but in an urban environment it's easy enough to stop by a supermarket every couple days on your way home from work/school. I can fit 2 supermarket shopping bags in the front basket and strap a bale of toilet paper to the rear package holder no sweat. Around here furniture and electronics stores typically offer same- or next-day delivery for free or cheap.

You can even do a week's shopping, using a bicycle-pulled trailer (otherwise used for carrying small kids). As a side-effect you even benefit from the courtesy of drivers, who may think you are carrying your children as usually.

Exactly. Weekly shops are a thing for suburbanites!

When you have easy walking/cycling access to multiple grocery stores, you just go shopping whenever you need something and never buy more than you can carry.

You can carry significantly more than nothing, but it's true that since I've started regularly needing to schlep sound recording equipment around cycling has become an increasingly impractical means of transport.

How often do you travel around town where you need to carry more than a backpack would hold, but less than what you can jam in a small car? For me, I could probably use a bike about 90% of the time. I don't always, but it's mostly because I'm lazy.

About once a week, maybe more. Visiting multiple stores (where it's already annoying to lug your prior purchases in with you) or just buying something like toilet paper will quickly achieve this qualification.

Even if cars were only used by each person a couple of times a week that's still a massive improvement over commuting everyday. (I also think you're exaggerating, toilet paper can easily fit in a backpack.)

Nearly all the grocery chains in the UK let you order online and have the goods delivered.

With a rack and a set of panniers, you can carry significant weight and volume.