Because back in the 1980s RAM chips were individually sold and the number of address pins exactly matched the 2^(# of pins) amount of memory addresses contained in the chip (for cost savings purposes) and only with the later column strobe addressing DRAM did that change but at that point they couldn’t deviate from the 1024 standard without getting sued (a couple did and went out of business).

And since the RAM chips themselves are stuck using 1024 and the RAM sticks are made of matching RAM chips (for cost and performance reasons); it will remain that way until people start buying systems on a chip with embedded RAM.