Tens of thousands of reservists have been mobilized. Under normal circumstances some of them must be abroad, others sick, still others can't be bothered and will find some plausible excuse to get them off, and so on. They're all in the middle of their normal lives, and would prefer not to be torn away from their studies, vocations, families and routines to go and get shot at by fanatic murderers.
The army knows all this, which is why reserve units generally call up a larger number of men than they'll really need, and balance the numbers.
The news broadcast earlier this evening (sorry, no link) told that more than 100% have responded, including many who weren't called but came; hundreds have been sent home.
Although additional units are still be called up. Meir called a couple hours ago to say he's been told to expect to be called later this week.
Bloggers who google until they find obscure sources that prove Israeli's resolve is flagging, take note.