Because they travel on the same surface with the same roughness maintained to the same level.
If cycle paths were precision machine flat steel rails you could have very small wheels, but since you must manage the same ruts and potholes you need a wheel of similar diameter.
There are folding bikes with very small (<20cm) wheels - but they are useless on anything other than perfect roads. There is a limit to how big you can make the wheels if you want a person to be able to sit on the bike (remember the penny-farthing. Finally large wheels on an off-road bike would be very heavy to be strong enough and on a high speed bike would have too much wind resistance.