Why don't swat teams ever arrive on time during school shootings?

the massacres at columbine and virginia tech are but examples of the tragedies that have unfolded throughout the last 20 years.there are many,many more.but i have noticed that whenever these shootings occur,the tactical police units that are "trained" to deal with these kinds of scenarios,always arrive AFTER the fact.you,when it's too late.the scenario almost always ends the same way-depressed bad guy{s} goes ballistic and shoots up the school;kills a bunch of people,and then turns the gun on himself. i understand that swat teams need lots of info to assess the situation before putting their men into harm's way.but c'mon,people's lives are being taken for every minute that goes by.these units are heavily armed,armored,and heavily trained to deal with these kinds of problems.they need to really step it up and cowboy the f*** up,if they are to ever prevent these atrocities from happening again!