why are built-in turntable rca cords so short?

If you want to run cables on a long distance you need balanced (XLR), not unbalanced RCA. Cable capacitance is critical for MM cartridges, the length of unbalanced RCA should not be longer than about 1 meter (this is not short for a home application). What is short for you?

Phono cables need to be low capacitance. 20pf/foot is a good value; certainly no more than 33pf/foot. By the time you have a meter you'll have 100pf or so; this can have a big effect on how a high output cartridge performs! That's why phono cables are often a meter or less.