I wouldn't say it doesn't offer E2EE by default. It offers private chats, which are E2EE, right there and almost as easy to initiate as the regular chats.
I used Telegram in E2EE mode with someone initially, but later we decided the multi-device sync and web chat were so much more useful to us, it trumped the desirability of E2EE and we switched.
Those features together would be better of course.
Apps like Signal and Wire have shown that multi-device E2EE is possible with messages synced across. While I believe that Telegram is far ahead on a lot of features, the lack of multi-device sync for “secret chats” is mainly because Telegram hasn’t spent enough time or effort on it.