Space, mostly. I don't have room to store all the stuff that would be nice to own if I ever need it. Some things get used all the time and I need to have on hand whenever I have a project. Other things almost never get used, and that's why God invented neighbors and rental companies.
Seriously, as a guy who works on his own vehicles all the time and spends a good amount of time doing DIY home stuff, there's certain things I need and certain things that are extremely situational. I need my wrenches, sockets, screwdrivers, drill, impact driver, circular saw, etc. as I use these on a regular basis. I'd love to have a planer, a jointer, a bandsaw, a sandblaster, A/C charging equipment, and any number of other tools, but most of the time I just won't be using them, so it's a waste of money and storage space. I have neighbors who have plenty of specialty tools and a Home Depot, two rental companies, an O'Reilly, and an AutoZone within a 15 minute drive. Anything I don't have and need for a specific project, I can get. That situation isn't all that common, so buying those specialty tools doesn't make a lot of sense because I'd be spending more money than the cost of rental (or a six-pack of beer in the case of my neighbors/friends) to buy a tool that I will use once or twice.
The other side of that is the snowball effect. If I buy a tool for a specific project, I think of all the other projects I could do with that tool, all the other things I could fix, adjust, or build. That inevitably leads to doing more complex projects, which will use the tool I bought more, but will also require buying more tools and doing more projects. It's a vicious cycle, and my time is limited.