"jestem bardzo tired po long dayu"
Now that's a bit over the top, innit (Polish translation: 'odjebało mu?')
why not create new words now?
Because they happen to be awkwardly lengthy, and they sound somehow ridiculous, now. Possibly it's easier to accept a completely new sound for a new thing, than to bend yourself backwards to derive it from 'purely' Polish material.
Now I look at my first sentence here, and I see it works both ways. It's perfectly legit to say in Polish 'no to już wygląda na lekką przesadę, nieprawdaż?', but my original translation is like more handy, and much more to the point, huh? :)
Edit: didn't refresh the page, my bad. Magdalena answered it much better.