Problems often slip through the cracks. Lines get jumbled. Computers crash halfway through scripts. Spacing gets screwed up. The lab accidentally changes words.
Cables go bad.
The final tape always has to be triple checked, just to be on the safe side. You won't have a second chance, and any mistakes will damage your reputation. With the black border taking up another color, that leaves two colors for the titles. By the time all is said and done, a typical movie will have about 40 hours of work put into it. I'll not point out incidents of bad subtitling here.