Why battery won't fully charge in Apple watch Model A1757?

If your battery health drops beneath 80%, the battery has to be replaced by Apple. If your battery has never been replaced, this is very likely the reason for the battery not charging completely.

You can get the battery replaced by Apple, have a look at this Apple Watch Service page to get an estimate for your model:

Apple Watch Service and Repair - Apple Support

Because it a very old watch. It's a series 2. The battery in an Watch slowly deteriorates over time and holds less and less of a charge.

M1 Mac mini, macOS 15;  Watch, series 7 GPS/cellular, watchOS 11; iPhone 12 Pro, iOS 18; TV 4th Gen, tvOS 18; HomePods, audioOS 18 and Airport Express w/ Airplay 2