Airtags and similar can work, but there are downsides. For one, AirTags specifically have a series of anti-stalker features. That means a thief knows the bike is being tracked. Second, any tracker you hide on the bike can be found by a thief and removed.
Having it integrated by the manufacturer means both avoiding Apple's measures and making it much harder to find and remove. So it's generally a preferable option when possible.
I think it would be hard to hide an airtag effectively on a metal-framed bike. Putting it in the frame would interfere with wireless communication and any other location would be easily discovered.
Having it integrated by the manufacturer means both avoiding Apple's measures and making it much harder to find and remove. So it's generally a preferable option when possible.