As someone who's been involved in both building and integrating billing systems, I can say you speak from a position of blissful ignorance.

Building the system always looks easy, and on day 1, it really is. It also guarantees you'll spend many hours with your Finance Director explaining why the reports you send them are complete garbage; many hours with your Support team explaining why your invoicing failed, why you charged incorrect subscription prices, any many more fun edge cases you never knew existed.

Next up, regulations change that you need to adapt to, or maybe your chosen gateway doesn't support a growing region.

And before you say "just build it better," remember: that's also time. Time not spent on your product, improving your pricing model (oh, you need to build that yourself, too, of course), or any number of things you want to do that actually grow your business instead of standing still.