You don't use backups because of privacy reasons, but the reason Telegram can save your data is because it's using backups.
I'm also not the biggest fan of WhatsApp, but a lot of its quirks and oddities do come down to one big design decision, which is that all your chats are e2e encrypted, with the key to the encryption living in your phone. This makes for (relatively) secure communication, but it has the side effect of making the UX in other areas suffer. For example, if your phone dies and you don't back up your chats, there's no way of getting the data back, because the encryption key necessary to unlock the data in the first place lives in your old, dead phone. Or the kind of irritating way that WhatsApp Web works: it needs to connect to your phone because your phone is the only place that your messages are stored.
If you compare this to Telegram, they can do a lot of this stuff more clearly, but this is because they store your messages on their servers, which means that you only need to connect to their servers to view the data. This is obviously a lot more convenient, but the cost is privacy: your communication can no longer be e2e encrypted because otherwise the Telegram servers wouldn't be able to act as a middleman. (Which, as I said earlier, is necessary for the UX experience you're after.)
This is a broad simplification, and there's a bit more going on than just this, but it's a good mental model to have when understanding why the UX of Telegram and the UX of WhatsApp are so different in certain places.
That said:
* "WhatsApp isn't completely e2e, there's flaws/loopholes/situations where not everything is encrypted" - this is almost certainly the case, I'm not saying that WhatsApp is the be-all-and-end-all of encrypted communication, and it is still a company that you're trusting with your data. Think of WhatsApp more like a bike lock: for most people it doesn't need to be perfect, it just needs to be hard enough to get in that it's not worth doing. If your actions make you more likely to be targeted, then WhatsApp may well not cut it as a tool.
* "You can encrypt messages in Telegram" is another valid point, and I did do this with my partner for a few years. In practice, the UX here is far worse than that of WhatsApp, and now we mostly use normal Telegram chats. This might have changed more recently, but a lot of the big issues (like not being able to see the chat on multiple devices) are limitations of the technology rather than implementation.
* "There are tools that do have end-to-end encryption but much better UX" - I am sure this is true, but at least where I am, the three main competitors are SMS, WhatsApp, and Telegram. I'm told Signal is pretty good, but no-one around me uses it so there's not much point using it. Hopefully with the new EU legislation around being able to sync up messages from different applications, this space will become more active.