If you improve the building, thus increasing it's value, you've increased the proportional value of this house compared to the market. Now, since this house has increased in value, the average price of housing goes up and this house is less affordable. If everyone does this then the same effect takes place of average housing increasing in cost but now there's a large quantity of very nice unaffordable housing. You no-longer increase it's proportional value and the average price still goes up. Your house is in the same class as other housing bought at the same time.
If all landowners continually improve their housing in order to increase profits the profits must increase to a point where the cost to improve is matched and exceeded. This means that all property one wants to profit off of must still increase in price quicker than inflation. Under your proposed solution this only means the housing is nicer.
The outcome is the same, less affordable housing. If more, cheaper, housing is created then the value of these improved properties must decrease as well (supply and demand) and these cheaper houses if the price stays tied to inflation the profit gained is stagnant. There is no incentive to increase housing.
Currently though this is not happening. Most investment properties increase in value without improvement. Most landlords simply profit off of ownership of this land and housing not in their added value. If all landlords were to improve housing instead of simply raising price then that would increase investment price while not increasing overall profit. They'd make the same amount of money they would if they simply didn't improve it. There is no incentive to improve as this adds risk for no reward. This is only possible on a small scale, and why we see few companies making profit off of buying housing, improving, and selling.
On rentals, even if they did cause value to be added, it would be through the money of the tenents and the labor of the workers. They are simply a middle man who adds no inherit value to this process. They produce no value, they simply take money from one hand, pocket some, and place some into another who adds value for them.