LSMFT wrote:

Seems like my dad had a machine that created (or condensed) oxygen
from the air for him to breath. No bottles to change.
Why can't they do that for welding?

Large scale O2 generation involves cooling air to liquify it, then pulling
off the components: Oxygen, Nitrogen, CO2, Argon, etc.

You CAN get Oxygen by electrolysis of water (plus Hydrogen), but the energy
expenditure is horrendous.

Certainly O2 generators can be powered by chemical means; the Oxygen masks
on airliners rely on chemical release of O2 by the burning of chlorates or
perchlorates.

All that said, you can get O2 generators for small applications (bedside,
veterinary, etc.) use, up to, and including, institutional generation, say,
for hospitals.

To answer your question directly: Bottled O2 is far cheaper than the
alternatives.