In an ideal world, everyone would be kind enough to do the repairs in time. And give back the house in the same (if not better) shape than it was when you started renting it. But we are not in such world, and people, tenants as well as landlords, will think first about their own benefits before those of others they may never know.

I'd be happy to not ask a rent if my tenant would repair and improve the house with their own money. But who would want to upgrade or repair with his hard earned money a house they may move from some years later?
That's kinda the main problem. The tenant do not have much insentive to maintain the house in good shape, other than for it to be good enough to live in. Once they want to move, they will just expect any repairs or improvement to be done by the next tenant.
But if the house is in bad shape (it happened to me after my first tenant who basically wrecked my grandma old house), no tenant would want to live there knowing they will have to pay a lot of money upfront to get an acceptable place to live into.

That why it is logical for the owner/landlord to be the one to take care of all that.
They are the ones who have the most insentives at keeping the place in good shape and improve it.
And to do that, they need to earn enough money, which cannot be done alone by working himself, thus what tenant pays: a participation for future repairs and improvements, in the form of a rent.

Now you'll always find greedy cunts amongs landlords, that considers the rent as their own pocket money. There are way to limit that kind of behaviour (rent limits, among others).

Add I didn't even talk about how to actually build houses. Where would you find that money? The tenant obviously doesn't have enough money, and so will the landowner without rent.