Motorbikes do not despawn when both tires are removed, provided the main vehicle body remains non-ruined. This allows stripped motorbikes to remain in the world indefinitely, bypassing the normal ruined-vehicle cleanup cycle (approximately every six hours).
The issue is compounded by the fact that motorbike tires do not independently spawn as ordinary world loot; they are currently found on functioning motorbikes. Consequently, restoring a stripped motorbike requires taking tires from another working motorbike, making reassembly counterproductive.
Over time, this results in motorbikes accumulating across official servers in a stripped, effectively abandoned state without ever being recycled back into the vehicle economy. This gradually reduces the availability of functioning motorbikes and prevents replacement vehicles from spawning.