It's already a thing, called house hacking. Tough luck if you're a couple that wants a complete unit instead of a room in someone else's house.
> a collection of people could own their apartment building together
That's called a co-op or a condo (the difference is a bit fuzzy to me). This still involves ownership and upkeep of an asset, which many people prefer not to do.
> huge companies owning large parts of the market
Make it publicly-owned, for all I care. Or let it be individual landlords. Or forbid a single company from owning more than 5% of the stock in a city. As long as it's market rate and abundant and competitive, it literally does not matter. More houses = cheaper housing.
> yet the largest landlords in the US was found to engage in price-fixing and artificially raising rents
Your own link shows that we know this happened because there was a Justice Department investigation about the practice. How can you say enforcement isn't happening.
And besides just having "real estate investors" slurping up all housing, people could own one house then rent out parts of it, or a collection of people could own their apartment building together, there are many other ways to make housing work that doesn't involve huge companies owning large parts of the market. A little bit of nuance goes a long way.