Well, thats also easy since telegram clients dont do much more than displaying messages stored on a server. Its more a viewer than a full client.
And that compromises hard on privacy and security, which Signal and Whatsapp dont do, they have proper Clients that have to really handle and store incoming messages. And the E2EE makes it harder, developing an independent desktop client, like Signal always had and Whatsapp recently got. But both are mediocre at best, sure.