Because like the (vast) majority of very successful people he started from a position of financial independence and social connections that absolutely are far beyond the norm.
Becoming very financially successful is very much more about what family you were born into and fortune than it is about skill or talent, although the latter certainly still matter.
Yes, they wrote an assembler/linker/simulator on old Harv-10. I was there bugging him late at night, asking why he was wasting his time on silly hobbyist machines when we had the whole Arpanet at our fingertips. (All 20-30 machines at that point.) ;/)
Even though lots of folks were using the -10 for commercial projects on the side, Harvard made him give all the Altair money back to avoid disciplinary action. Wasn't really fair.
So, yeah, some of us were blind at the time. In my defense, I did help Carl Helmers get Byte Magazine off the ground a year later or so, along with my high school buddy Dan Fylstra who went on to create VisiCorp to commercialize the original VisiCalc.
My main question is, how could Bill Gates afford the $40,000 of computer time? That's quite a lot of capital at the time.