It's a commons problem. In your scenario, you have absolutely increased the value of your property. 20 apartments is more valuable than a single family home. However, you have also reduced aggregate demand by housing 20 people. That reduced the value of your neighbors' properties (or at least reduced the rate at which their value increased).

NIMBYism posits that values will continue to increase without forcing any property owners to invest in improvements so long as they collectively block any efforts to increase supply.

Now we have a large homeless encampment partially on the apartment complex lot - that the complex can't deal with because their lawyers told them off.

Now we have a significant uptick in petty but quality of life impacting crime.

The roads around the apartment complex are now swamped because the entrances were placed stupidly.

And the people who live in that apartment complex are also upset about all of this.

None of this is about "my investment". It is about not letting commercial outfits hurt my quality of life to subsidize their profit margin. It's not the fault of the tenants, nor me.

Now we get to sue the stupid property management company until they fix their issues.

The thing that is prized above all else is privacy and exclusivity. People are paying money to ensure that few people live near them and that those people are the "right sort of people."

Dating back to the 1930s the original core goal of zoning was to restrict apartments so as to keep poor people and minorities out of certain areas. We know this because it's well documented in the newspapers of the day because back then people were more comfortable explicitly saying how classist and racist they were.