Some games just have *their* music.
It is not just *soundtrack* or something that player should *listen* while playing, it is more a very deliberate design choice made in order to create all-around product.
Such examples can be traced at FF series, MGS series, Silent Hill or Siren series, as well as many other titles that are heavily influenced by wish to create some virtual universe.
As we talk about GT series, you have to look all the way back to the first game, take some cultural implications into account and also accept how Kazunori Yamauchi and his closest team of associates are trying to create a virtual universe that will have some unique spirit and carry some emotional impact onto players.
Notice I will not so much discuss "international" releases of the game, because of the one simple fact - both US and Euro soundtracks in every GT game so far was heavily influenced by SCE producers at particular territory. Thus the vast difference between GT3 US and Euro soundtracks for example, where regional SCE GT producers opted for songs they found "important" for particular territory (probably based on input from Sony Music division).
Another really bad example of above are changes made for opening themes for every GT game so far. You probably know about famous Moon Over The Castle theme, which is the trademark of the whole Gran Turismo series - same as "original" MGS theme (despite it is blatantly stolen by Tappi Iwase from Sviridov's Puschkin Garland overture) is for all MGS games (exception is MGS4 because of the copyright issues Kojima had with Sviridov copyright holders). That original Moon Over the Castle theme is used on opening intro of every GT game so far, but US and Euro producers never allowed it for the US/EU releases of the game. Result is completely butchered intros of all GT games so far - with only exception being famous GT2 Cardigan "Loosing my Favorite Game" intro that was somewhat made bearable - and greatest butchery being made for GT5 where they've opted for non-inclusion of original MOTC anywhere on the disc which I still can't comprehend.
First GT game had a great soundtrack from Masahiro Andoh and Isamu Ohira which was a tribute to Japanese heritage of 80's heavy-rock genre being associated with the driving genre. Notice how the first GT game was also the most profound soundtrack-wise because each manufacturer and section of the game carried it's own theme which was extremely beautiful detail and that was the place where Andoh and Ohira experimented with pure electronica. Since the first game Ohira is the man in charge of menu music.
GT2 also carried their work and GT3 introduced another great composer, Daiki Kasho. He continued his work together with Andoh and Ohira on GT4, creating some of the best songs for the game and GT4 was also the first to introduce a classical selection of the music for Photo galleries.
GT5 pushed the philosophy into further universe, with 189 songs and some of the best Japanese authors on the soundtrack through various genres. All of them are probably very carefully selected by Yamauchi and Ohira, both great lovers of classical music and contemporary jazz.
You may not personally like the music they are using for the game, but you should really try to *understand* the logic behind their choices. What Western people often hear as "another elevator music" is what Japanese contemporary-jazz authors currently experiment as of the most important genres for their musical scene.
If you take a time and explore the whole logic behind deliberate choices made for soundtracks in all GT games, maybe you would better understand the very clear and logical path of development of the unique and recognizable atmosphere of Gran Turismo series soundtracks. Also, no game except Gran Turismo can say it had the world's best piano player willingly opted to play for it's main theme and record his versions of some best classical pieces around for it's soundtrack.
What you find embarrassing - and you are really free to feel so, it is 100% your private opinion you're free to express - is probably just result of not being really into music and heritage of GT series philosophy. I really hope that one day you'll be able to comprehend and favor what Yamauchi and Ohira are doing for last 14 years.
For more information about actual changes for international releases of every GT game so far and vast differences of official soundtracks, here is the useful link:
http://en.wikipedia.org/wiki/Music_of_the_Gran_Turismo_series
Regards.