Because PCs were originally business machines
Originally, they weren't. The first IBM PC shipped with 16KB RAM, a cassette tape interface, and BASIC in ROM. They accidentally failed in the home computer market, and succeeded in the business microcomputer market. They then tried
again for the home market, with the PC jr. but failed. The PC jr. didn't really sell until they introduced a proper keyboard and an upgrade module that enabled it to be used for the same stuff that PCs were being used for.