Because building for my family's future is one of my primary motivators. I'm not just in it for myself. If I die I want my kids to receive the same education they would if I were still alive. The government already takes half of what I earn, and I will do everything legally possible to give them nothing more when I die.
Maybe the real issue is that your goverment isn't actually providing education for your own children. Your dependency on being able to leave capital to you descendants is merely a symptom of that actual problem.