Because it was never meant for that purpose. RS232 is meant for simple easy communication in friendly conditions. Like connecting a printer, connecting a simple dev. board over 2m of wire to a PC to send a program, to attach a modem that lies on a shelf to a PC under your desk. Its advantages are that it's simple, it's cheap, it's easy to use, the UART chips and data lines are simple and very tollerant to different cludges. It's prefectly enough for home or for your developer's workshop or to flash the memory of a cellular phone. It's used in applications where a failure of transfer won't cost more than, say, $50 as flash of the phone will get corrupted and your phone will need to be replaced. It's not very reliable - but it's cheap.

485 is meant for different purposes. It's where a failure could go into billions when an oil refinery blows up because some temperature sensors didn't detect the oil overheated to burning point. It's much more reliable, it's much more "heavyweight", it's much harder to get done right and much more expensive when done right.

If you're going out to a jungle, you need a machette, your butter knife won't suffice. And you don't need a machette to smear butter on your bread.