Because it should be the responsibility of the landlord to maintain the property. The users are paying to rent a space within the building. They are not responsible for the building itself, outside of property damage. Property maintenance is the responsibility of the person renting out the building.

why shouldn't the users who use it the most pay more than those who don't?

Why should any users be paying for a common public area that they have no responsibility over? When the elevators break down do you think that the landlords refuse to have them repaired and go around door to door asking the tenants to chip in? Or do you imagine that they will lower the rent if tenants have to pay to use the elevator because that subsidizes their operating costs?

Should they also charge users for entering through automatic doors? Those wear down over time, and they could put the money towards servicing them.

What about a fee for using the stairs? They have to replace the non-slip tape on them every once in a while. And for VCT stairs, the landings need to be stripped and waxed to maintain the protective coating. That is another form of maintenance that is affected by wear and tear. So obviously those that use the stairs more often bear a heavier burden on the upkeep.

Or should people be charged a fee for walking on carpeted hallways? The carpets will get dirty over time from people walking over them, so by having everyone that walks on them chip in, it keeps things fair when it comes time for the property owner to get the carpets cleaned.