Because most companies/startups have this in-built assumption that they will eventually grow like wildfire. Obviously this is because they are selling to investors who want this to happen so they can cash out on their unicorn. So they just build it in the cloud in the first place with that in mind.

Now in some miniscule amount of cases this is true and probably did help some people who's business 100x'd overnight, but in the vast majority of cases your business just will never get to the point it needs to be "cloud scale" in the first place. Nevermind accidentally shooting yourself in the foot with a recursive lambda here and there in certain instances or a misconfuguration causing a huge bill.

Edit: Another is because lots of companies who do actually end up succeeding negotiate a shit-load of credits with cloud providers so they can basically grow their business for free for a while. That is until those credits run out and they get hit with the actual costs.