Maintenance cost for Motorbike vs Automobile
The OP is merely asking "why" it costs more to maintain a motorcycle more, and to answer that question properly is first we need to establish how are you using the bike. And what is important to you when riding a motorcycle, this is based on how you are using the bike.
If you are using it as your main source of transportation, then you are really concerned, IMHO, in getting from point A to B. You may not be too concerned if it's parked outside all year round. This is similar to most car drivers, hence, you don't want to necessarily maintain the bike or upgrade things if you don't have to. You may pass on the 6,000 mile or 12,000 mile service and do it when it is really needed.
[LIST][*]It is quite true that motorcycle is a toy (you don't need it, cause everything you do on a motorcycle can be done on a car and do much much more).[*]Motorcycling is a sport[*]Most people also fall in this category - Motorcycling is a hobby.[/LIST]
When you look at those three items, cost to operate starts to become of a secondary point, while the fun and safety starts to take precedence.
As someone pointed out earlier, maintaining a high performance vehicle will cost more if repairs are required. Now why do we follow the prescribed schedule is because we are scared that what if it breaks when you are hundreds of miles away from home. That is not fun, in fact, it rained on your fun event. No we don't want that to happen, hence, we take precautions. You then follow the gurus (dealership service advisers, BMW, anyone who tells you something from their experience, etc) and spend the money so we minimize failures.
You also do it because you want safety. Someone tells you if your tire pressure goes down and you chances xxx could happen, you go get TPM, if they say traction control will save you from possible fallout, you go sell your current bike and buy new with Traction Control, etc. We are scared and we don't want accidents, 'cause it hurts, hence, you will spend the money to rectify this. You don't care this if you have a car.
You spend money farkling, why? 'cause you want convenience! Extra lights for safety. Engine guard - in case you drop. Rain proof gps vs regular. I bet if I were to offer a rider a garmin weather proof gps and a nuvi at zero cost, you know which will be the ideal pick!
There, in a nutshell you spend because you WANT to, and not that anyone is imposing on you. And your reasons for "want to" can vary on your needs and requirements and how you view riding a motorcycle - commuter, enduro, touring, racing, or any combo.