Very simple; at the prices you cite, the largest charge is the one you forgot about: people hours needed to maintain and setup these services. 100$ sounds like a lot but it's only 1-2 hours of development time per month; less if you are in a competitive market. And lets face it, it's never just 1 hour per month. Just spending a week or so setting this stuff up could fund years of SAAS bills potentially in terms of time spent. And a week is not a lot.

You also get this sunk cost fallacy where you basically keep on poring in more time (and money) because you've already invested so much. Self hosted commodity services only makes sense these days if you have too much money or very specific needs that you are willing to pay for. Even at their premium pricing, they are a pretty good value proposition for most companies.

Asana on the freemium layer is still pretty good value and an easy upgrade once your team grows. Same for github. We pay for neither. Github comes with Github actions too with lots of free CI minutes per month. The Slack freemium layer is pretty decent as well (more than enough for any startup). I'd consider paying for all of the above if I had to because these products are kind of nice to use as well. But I don't. 0$ / month is a pretty hard price to beat.

But even at hundreds per month, self hosting would still cost more and deliver less.