Mostly cause the satisfying functionality/usability and least laziness to move on to another platform. Also I am subscribed to many news, software and dev channels on Telegram so I just sticked with it. Also I use AdGuard on my phone and I never saw ads in my Telegram; was shocked seeing ads pop up in my friends Telegram though. But off topic here, I think Z Library's channel being on Telegram was not a good idea, Matrix would have been a way more robust option. But there is worse: pirate servers run on Discord, like seriously... why...