I don’t, of course – why would I?
Because it’s vaguely whimsical and bikes and cars have their own character and resonance in your life I guess. No-one’s saying you have to, but if it gives someone pleasure, then that seems fair enough.
My road bike is called Mog, partly because it’s a fast kitty and partly because it dates back to a particular point in my life and reminds me of someone.
I guess personalisation of inanimate objects, maybe non-living objects, is slightly illogical, but it’s maybe a reflection of the emotions they stir in you. Or maybe it’s just shorthand to identify one of several bikes. But basically, why not?