Also, building in-house isn't that expensive. Or at least, not compared to developing in-house. I'm not all that technical, and I setup my own Docker registry, for example.
I am one of those people who only allow minimal required JavaScript :)
Can I build a website? Yes. Can I save money by paying someone to build a website and focus instead on filling it with great content? You bet.
Same applies for SaaS. It would be a waste of time/money/effort to build my own ConvertKit for example
There's also the issue of sharing sensitive data with SaaS providers. Just think of all the data leaks that's caused. The Squarespace heatmap stuff we just saw. The deals between Google and phone manufacturers. The behavioral-ad-delivery morass.
Re ConvertKit, the alternative would look like "git clone foo".
I like that I can pay for software based on how much I use it rather than ponying up hundreds of thousands of dollars once. A lot of the cheaper end SaaS-es could never ever be sustainable without the monthly subscription business model. Particularly because they themselves use SaaS and have to keep the lights on.
Imagine if isntead of paying a monthly fee to use whatever services companies use to run their company they'd be required to build everything in house? Ain't nobody got the funds for that.