For the same reason I order a delivery rather than collection on a takeaway or get my shopping delivered. Because I weigh the amount of time and effort of going somewhere against the contents of my bank balance and decide if convenience is worth the additional cost.
I am a self confessed lazy bish though and much prefer offering incalls from home (and getting stuff delivered to my door!)
There are many, many reasons a client might want an outcall, maybe they don't have access to a car, maybe they're not local and don't know the area, maybe they have mobility issues or anxiety about leaving the house, maybe they're high, drunk or hungover, maybe they have time constraints and can't afford the travel time both ways or maybe they're as lazy as I am and just want the convenience of being able to relax once the sessions over.
If you don't like outcalls either remove them from your profile or charge an extortionate rate for them, take a hefty deposit and have stringent vetting in place. The deposit should be enough to cover all transport costs both ways + enough that if he cancels you'll have money in your pocket. The rate should be enough that you can put on a big smile and provide a great service even if you've travelled hours through less than ideal conditions.
If you do those things and guys still hassle you for an outcall at a lower rate, then block them and come have a moan about them in the "How not to book an escort" topic.