Actually, at least as of a couple of months ago, if you are generating all new coins, you can prevent all transactions from being validated (by refusing to include them in your blocks) or undermine community faith in bitcoin integrity by double spending. Both capabilities would be pretty devastating.
The main reason a government would want to do this is that control over the coin of the realm is a major source of state power. If a government ever concluded that bitcoin posed a credible threat to that power, that would be a strong incentive to undermine it.
You think NSA or GCHQ[1] don't have the knowledge to implement an efficient bitcoin machine? You think they don't have the money? You think they don't already have significant computing power?
Or are you suggesting they'd be hampered by legal oversight issues?
[1] GCHQ probably have the most powerful computing facility in Europe. That puts them amongst the most powerful computing centres world wide.
