Because even if we withdrew completely, I believe they would continue to target us for terrorism. They hate our free market. They hate that their populace loves our culture. They hate the example we set. They hate our wealth. And most of them need someone to hate so they don't have to face their own failures. If we withdrew they would still rant about the past, demand apologies in cash form.... indeed, I wonder if they wouldn't hate us even more. There are a lot of places only barely under control and if we leave, all hell will break loose. And who will they blame? Themselves? Ha.
Now, you may say, "Fine, let 'em hate us, what can they do?" But we've seen that they can do plenty. Our borders are very porous. We'd have to clamp down at the Iron Curtain level. I can sympathize with your point of view and I think that if we did indeed clamp down HARD on immigration, it might help. But they'd still do their best to hurt our business interests overseas.
No, I think it's 70 years too late for military isolationism, tempting though it is. Communism, Socialism, and Islam are all spreading like cancers, and a huge capitalist country is a major impediment to all their agendas. They have to destroy us in order to complete what they all see as their Manifest Destiny. They have already indicated their willingness to band together in order to try and bring us down. (If they succeeded, of course, they'd turn on each other in a manner that could really cause the War To End All Wars, but that's another story.)
Before 1950 the USA was a loved and respected country. We minded our own business and tended to our own affiars. (Excluding Central/South America were we have always meddled). I can somewhat for given our leaders for being interventionist during the cold war. You do what you got to do. But there is no excuse now and we would be wise to return to our pre Cold War policy of intervention ONLY when American lives or property are at risk.