I wouldn't call it helplessly outdated, at most it's a version behind. But there's a number of reasons for this. For starters Telegram themselves lag behind in releasing the source code for the app[1]. Then the volunteers who make the FOSS version[2] have to update their fork and make sure it works. Then finally it's submitted to F-Droid, where it's compiled by their build servers (of which the build process occasionally breaks and needs to be fixed).
I think the assurances that this offers (entirely FOSS, reproducible builds, better privacy and security[3]) outweighs being sometimes days or weeks behind the releases they upload to the Google Play Store.
