>Give me an app, mobile or web, and I'll find a glitch or two in the first few minutes of using it
I find that a very strong claim, is it rhetorical or meant literally? If the latter, what can you find in (say) whatsapp* ?
(*: aside from messages delaying or appearing out-of-write-order, which are probably backend bugs inevitably arising from its distributed nature)
I deleted WhatsApp a while ago so can't say right now. It is a pretty mature app that hasn't added big features in years I think, so I wouldn't expect a lot of bugs in it, but take a look at their version history on App Store, there are plenty of purely bug fix releases only this year.
A front end is a state machine that relies on a bunch of enormous framework layers. Give me an app, mobile or web, and I'll find a glitch or two in the first few minutes of using it.
Backends on the other hand, like you said, only ever talk to other programs. And that means building a 100% correct and bug free backend system is not such a fantastical goal.