Because it looks easy, and the vast majority of humanity overindulges in narcism and wishful thinking.
Everybody also thinks they can write - but spelling, grammer, and a good vocabulary do not a writer make. That takes having something worth saying, and some skill in saying it well. Everybody thinks they can drive. Everybody thinks they’re great in bed.
With most things - it takes thought & hard work, just to get started - and it takes a lot to write a first draft, or build a prototype. With software, you can pound keys, move fast, break things, and fix them later - assuming nobody has died from the errors.