I don't know about the Emirates, but Spurs new stadium has a bit of parking underneath but not for general admission. It's for disabled fans, staff etc. Parking on site is pretty much essential for disabled access and no one will argue with it, but you are unlikely to see stadiums built with American style parking over here.
The problem with building parking spaces is you run into the principle of induced demand. You think you solve your traffic problems by building more car parks, or add an extra lane to the motorway. All you actually achieve is increasing the amount of cars in the area, increase the amount of traffic and create an even bigger bottleneck further down the system.
Which is another reason I can't ever see a large parking development near Ibrox. As much as we would all love to jump into our car and park right next to the stadium and stroll to our seats, so would 50,000 other people. The football traffic is bad enough without adding potentially thousands more cars onto a road network that can't deal with it.