Plenty of hams do use their towers as antennas:
The magic words for Google are "shunt fed tower". Shunt feeds are most common, since most masts are grounded, and you can't series-feed an antenna if it's grounded at the base.
There certainly are towers that are isolated from ground, and are series fed, but securely attaching a tower to ground without connecting it has obvious disadvantages to cost and complexity. Consequently you will probably encounter them only among the most avid 160 and 80 meter users. W8JI is a good example. For higher frequencies, a quarter-wave vertical can more cheaply just be a tube, since the structural demands are less.
As for why people put HF antennas on top of their towers that could be perfectly good HF radiators: usually the antennas at the top are directional: Yagis, quads, etc. Feeding the tower gets you an omnidirectional antenna which is great for broadcast, but not for DX hunting.