I would LOVE to be a tool developer, but...

developer faces a problem as part of normal app development
developer fixes the problem
developer faces the same problem again.
developer fixes the problem again.
developer faces the same problem 5 more times
developer builds a tool that automates the fix

time passes

tool gains popularity
developer now is a tool developer and spends all his time on the tool

time passes

tool gains even more popularity
developer is now part of (or owns) a company (or opensource project) whose product is the tool

another developer has a problem in his app domain that should be a feature on the tool
unfortunately original developer will never see this because his domain is now the tool itself.