>What possible reason is there for them not to just be plugged into the car's power and computer?
The part where they are a sensor in a wheel and therefore have constant turning. Are you interested in engineering a system that can cheaply and reliably provide power and signal through a constantly and one direction turning joint? That's not a trivial problem, and most solutions are things like contact brushes on a turning bearing surface which would instantly foul in a tire and brake dust filled environment or a sealed puck of mercury channels that nobody wants to install on every single car in the world.
There are two ways tire pressure monitoring is done. The normal way is to piggy back on the tone wheels that ABS uses to monitor wheel rotation speed, as a flat tire has less circumference and therefore rotates faster. This has the down side that you need to "calibrate" it and people suck at doing that, it can't tell you raw pressure values at all, and for a while it wasn't normal for cars to have 4 independent ABS tone wheels so you couldn't always pinpoint which tire was flat. This method has no consumable parts, has no batteries, and sends no radio signals so is not trackable.
The other method is putting a battery powered pressure sensor and radio in the valve stem of each wheel. This method is retrofittable, will always give you raw pressure values and doesn't need any calibration (but does need pairing). However, the parts are more expensive, they are somewhat consumable and make tire changes more expensive and time consuming, and are constantly sending trackable signals that can be automatically dragnet surveilled. Don't buy this method.