Because even if they look for it and find it, whoever is riding just says it theirs and there is literally nothing the police can do unless it was caught on video or there is a meaningful identifying feature like a serial number or something else specific and unique.
Seeing a sketchy guy with a black and red bike with the same bike rack you had isn’t enough to prove anything.
If an officer approached me riding my bike around and asked me to prove it’s mine, I couldn’t either despite not being a thief.