You just haven't experienced them. Okay, let's take your statement for a moment. Windows 98 had something like 16 million lines of code and plenty of errors. Vista has something like 10 million lines of code more than XP, and probably about 50 million lines total.
Let's assume that this code is 99.9% errror free (which it isn't; the typical estimate on errors is much higher). This means it has *a minimum* of about 50,000 bugs in it. Huge programs have a very high probability of error.