There is some debate about whether it's a good rule in the first place but there is a number of reasons why governments don't do this:
Because they don't have to.
Governments have in their taxpayers a captive audience that always has to pay; limited only by their willingness to pay. So as long as the population is not too upset by the tax burden and the level of spending, they will continue to spend a lot. Why?
Because they don't want to.
Spending money is a lot of fun and brings you many friends! More importantly, it helps governments to be seen as 'doing something', creating jobs, saving puppies, fixing infrastructure, putting their name on things, preventing
<insert great evils>.
There is an unlimited amount of good things a government could do if only it had enough money. Also...
Because it's really hard.
Besides popular perceptions, governments are under pressure by well-organized lobbying groups that want something and will finance the opposition if they don't get their piece of the cake. Cutting back on expenses is highly unpopular and a government that doesn't do enough about things will lose popular support. Those that promise more stuff are likely to become popular in their place.
Because there are no good times.
Times are never good enough to stop spending. Even if one crisis is over, there is always unemployment, wars on
<something>, environment, energy, potential alien invasions, etc.
For a politician, the best place to be is in a crisis seen to be doing something, or at least criticizing those that are not doing something. It doesn't matter whether the crisis is real or not.
Why don't governments spend more during a crisis?
Option 1: because there is an opposite rule that suggests government should seriously cut back on spending when they have a lot of debt, to ease the burden and allow the private sector to recover
Option 2: because the austerity programs are mostly a sham with token reductions in spending but overall same or more money spent
Option 3: because the governments are already close to the limit of what they can spend
Option 4: because in order to borrow more or to get things working they actually have to reform
Option 5: because they are 'doing something' in the other direction of less spending or are playing politics with it (the evil IMF/EU/banks are forcing us spend less so we have to cut back on your favorite programs!)
Any one or more of these could be true.