Same reason the wheels on a shopping cart are small; they’re there solely for the purpose of rolling. The wheels on a car are what moves the car. An airplane uses it’s propeller/turbofan/jet to push it along the ground, so the weels just need to be there so it can roll.
There are exceptions to this, of course. Airplanes designed to land on dirt and in fields have much larger wheels, and take a look at the back wheels on the landing gear of a Naval aircraft designed to land on an aircraft carrier. They’re much more . . . robust. They need to be to handle the impact and/or the rough terrain. Most airplanes, however, are designed to roll across asphalt, concrete, etc.
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