Google has good lawyers, and their terms are both brief and written in plain English with no "fine print".It doesn't matter how many lawyers they've got. For one thing, in most places in the West, if you're in a small claims court or the equivalent then professional lawyers probably won't be allowed. And if your case is large enough to be heard in full court, it's worth hiring your own lawyer, and they're going to drive a pretty obvious hole through any contract of adhesion where the terms are essentially "We can decide arbitrarily that you're in violation and thus refuse to hold up our end of the deal".
...none of them have invalidated the contract in a courtroom thus far AFAIK...
Are you familiar with the case of Aaron Greenspan?
http://www.huffingtonpost.com/aaron-greenspan/why-i-sued-goo...
http://www.huffingtonpost.com/aaron-greenspan/why-google-bot...
The key facts as reported appear to be that he did take them to small claims court after they shut down his account and wouldn't explain why, and he won. They then appealed, and they won, but only after identifying two specific ways in which they considered one of his sites did violate their terms (at least one of which is explicitly identified in the wording they use today, though I don't know whether either or both was at the time) which is all he was asking for in the first place.
Net result: Google were probably out far more in time and money than he was, and got a healthy dose of bad press for their behaviour throughout. The lawyers might call it a win, but I bet neither the accountants nor the PR guys did.