I am not going to go into every nuanced issue, but to give context rounding errors alone costed us hundreds of thousands of dollars in customer support and eng time. I am talking Office Space style fractions of pennies type problems. Mostly due to how long it was an issue before it was caught.

Sure we could have probably botched implementation of some vendor, but we made every dumb mistake you could make and instead of doing it slowly we did it at scale.

Because they're actually the same thing. A team that thinks it's the right thing to build their own billing system lacks the experience and knowledge to do it correctly.