"Your" chat consists of other people's messages too and they own those. You can also delete your messages on Discord or Slack – it's quite normal as a feature.
Of course, I have nothing against being able to delete your own messages for both. A time limit within which you can do that is nice but I can live without one. But Telegram lets you delete every message in any PM conversation, including incoming ones, for both people. Or even the entire conversation, and make it disappear from the other person's list without a trace.