Consider the bridge example brought up earlier. If your bridge builder finished the job but said: "ohh, the bridge isn't secure though. If someone tries to push it at a certain angle, it will fall".
All bridges have certain limits. There is difference between a footbridge and bridge that can be driven over with a tank. The difference is also reflected in cost. You are advocating always building "tank" bridge. Which is understandable attitude - this way you are mostly safe. However, in some cases it is *economically feasible* to just build a simpler bridge and accept the fact that it will break under some conditions.
Ultimately it is a matter of economics. Sometimes releasing something earlier is worth more than the cost of later patches. And managers/customers are aware of it.