Because companies don't care about how 'clean' you code is, what design patterns you use or what cool new technology you are using. They just care that you manage to release quickly so the company can start making money.
So even if you don't know the best way to do things or the best pattern to use, as long as you can hack something together that works reasonably well for some time then you're good enough. Sure, it might be flimsy, break easily, unreadable etc. but if the company is making money then business is happy.
Think about it...code becomes legacy so quickly. 2-3 year old code is considered legacy, especially if it's not used/modified much. You could have spent 15% more time to make it more maintainable but it will still be considered bad code by the next guy who had to work on it because he has absolutely no idea how the code works and what it is meant to do. Chances are, it would be re-written anyway even if you spent that extra 15%. The in the meantime the company might have missed their initial release date and some potential revenue.
Not saying I agree with it but that is how it is unfortunately.