Many reasons - I like biking, good for the environment, get my exercise in (too much sitting at my day job), saves money, etc.

Some of the great things about biking are how versatile, liberating and humane it is. I can hop on the road and ride fast for vigorous exercise / endorphins. I can take a rail-trail or MUP and ride at a more mindful pace while I observe the ducks, trees, cars, people... I can hop off the bike and walk for a bit on a sidewalk if there are barriers to bike travel. I can make eye contact with other travelers. I can wave hello to people I see regularly. People sometimes ask me directions at stop lights or I chat with other cyclists while we wait for the light.

I'm a big fan of the author Paul Theroux. In his travel books, he writes a lot about why he prefers "overland" travel over air travel (overland being most forms of travel where you actively see what you are traveling through). A plane is faster than a train, but you mostly see a layer of clouds below the plane. A car is usually faster than a bike, but you need to have a certain focus on other cars and road users, which detracts from your ability to mindfully observe your immediate environment.

Traveling by bike, however, strikes the perfect balance between being able to mindfully travel through the environment, while also being able to reasonably travel to places within a much larger radius compared to what you could walk to. Commuting as passenger in a bus, car, train, etc. is preferred over driving due to the ability to freely immerse yourself in observing and "being" in your immediate environment. But, you have much less freedom over when you travel, how fast you go, spontaneous route changes, etc. You might also have issues with people feeling that you stare at them too much if you are sitting in one place on a bus or train.