myself, unless I REALLY REALLY don't have the time to do it myself. My yard is small enough that I can usually find an hour or two a week to do it. Plus, I feel like I probably pay more attention to detail than a crew that I might hire. Also, as you mentioned, I consider it part of my weekly exercise. And it's also a time I can put on the ear plugs and kind of be alone in my thoughts.
But $4k for a lawn mower, unless it it is a riding mower seems like an awful lot. And if it is a riding lawn mower, then you're not really getting much exercise.