Telegram does have E2E encryption if you toggle secret chats, it's just not by default because most users don't care about privacy and just want the cool features.

In 5 years of using telegram, I'm yet to see a person who would initate a secret chat with me. Everyone who loves Telegram hypes how cool it syncs all their messages on all devices, including browsers, yet, being happy of using the most secure end-to-end encrypted messaging. Sad.

Explain in technical detail why the cool features (like stickers) can't be implemented with E2EE when Signal is doing that at constant pace?

Telegram doesn't have E2E encryption for groups or Windows/Linux desktop clients, even if you try to toggle something. So your argument is false.

Stickers do work in Telegram secret chat, though. Only animated stickers don't, presumably in order to avoid leaking IPs. But a lot of cool features do work in secret chat.