This is actually a quite complex topic, because it isn't as simple to catch many of these bugs as one might think.
Software is very complex and when pounded on by thousands of people, almost all with individual computer specs etc, it is all but impossible that something doesn't creep up.
It is almost impossible to test any complex piece of software on any working PC for every scenario of users. That is why big MMOs usually have a long beta period for example, to get "free" testers.
Secondly as also mentioned Siberys there is the deadline, money aspect, of the problem. It is expensive to develop this type of software, and many companies do run out of money and thus can't survive unless their product is released at a specific time. This can have unfortunate consequences for the stability and bug issue, but the alternative could be no game, or no company.
Also there can be economical interest from backers, publishers etc which will attempt to get the game pushed out, to get a return on their investment.
So the problem is much more complex then just "why don't they do ...." because that is rarely an option.
Outside games mentioned, the "famed" MMO Vanguard originally made by Sigil is an excellent case study on how bad it can go actually if a company does not seem to control their process.
However that being said- I do agree some companies will throw just about any state of game out on the market, and many games are embarrasingly unstable and bugridden.
However, my blame falls more upon the consumer then it does the company. They just make the stuff - it is the people who buys the products and thus reward such release that I'd rather have a few words with
I hear so many people complain about bugs in newly released games almost everywhere from singleplayer, multiplayer to MMO. Yet, these people have bought the game themselves without (or in spite of) checking if indeed the game is stable, bugfree etc
If people stopped buying bugged games, developers/publishers would be forced to polish their games better up front.
Being reported as buggy was for instance that I stayed away from Dungeon Lords, and why I never tried out Vanguard.