Because if you want to be a software developer in the long term, you need to prefer the long term alternative in most cases.
A typical example is, "If we don't get something out the door, we'll be out of business. 'Shit' is something that can be shipped quickly, therefore we must ship 'shit'."
But companies that ship 'shit' generally go out of business anyway. Either their customers find it unappealing and leave, or ongoing maintenance quickly becomes so difficult and expensive that the product can not improve except by being rewritten under new management.
With something like a secure website (or script injected into arbitrary websites by a large ISP) the severity of the security vulnerabilities that tend to result from "shipping shit" often you only get one or two chances as a company.