Personally for me it is about the traffic that may be routed through my computer by the Tor network - I definitely do not want child porn, drugs or terrorist related site transactions packets to even touch my computer. It maybe a rare occurrence, but I want certainty. If we could control the traffic that is allowed on our network / computer, I'd be a more willing user of Tor. (A use case example would be to allow a Tor user to create a white list of onion sites from which they would be willing to accept traffic).
Not only that, but TOR is something that becomes more private the more someone uses it. Iirc it's really easy to distinguish TOR packets from regular packets. Combine that with how few people use TOR, you're job of narrowing down who's abusing it becomes much easier.
Finally, just running an exit node opens one up to many legal liabilities. It's not something that's worth the effort, but it's strength comes from many people running one.
No traffic is routed through your computer by using Tor, running a relay is a completely separate thing that can't be done by accident.