The reason they have had bugs is due to a shitty publisher, PB is clearly very very talented as a group of about 16 turn out these incredibly complex and innovative RPGs.
You couldn't be more wrong.
Making games is a collaboration between developers and publishers. Publishers don't go around forcing developers to start projects they cannot complete. On the contrary, developers start on projects, and then usually run around and shop for publishers, proposing features, timelines and budgets. More often than not, developers propose extremely optimistic budgets and timeframes, hoping to get a publishing contract, and then they end up incapable of making a quality game under those conditions.
Cases are extremely rare when a publisher rushes in and releases the game before the contracted release date, not giving the developers enough time to finish. What usually happens is the opposite: the developer promises the earth and the sky, but then when the release date comes, it turns out they were only able to do half, and even that is a buggy mess.
So the reason Gothic games are buggy is because the developers bite more than they can chew. They cram features in there that they cannot get to work in time for release. It's hard to fault the publisher for not saying "oh hell, you promised to deliver this game tomorrow but you couldn't and it's an unstable buggy mess, well, fine, here, take another couple of mil, go on vacation, finish the game next year!"
Buggy games are caused by poor planning by the developers. Sometimes they promise too much because they're dishonest and just want to screw the publishers, and sometimes they promise too much because they're eager and optimistic and don't know what they're getting into. In Gothic's case it's clearly the latter, but with a bit of a former too, because honestly, after all these games, they should learn to plan ahead already and be able to figure out which features they're capable of delivering in time for release.