Because it's not just AIPAC! The entire military industrial complex is making a killing selling bombs and guns and jets, and those pro war voices are incredibly powerful and influential in both parties. Blaming AIPAC misdirects our attention by blaming foreign influence for a home grown probablem.
Wherever AIPAC is present, you can bet the Pentagon and all it's supporting industries are goose stepping in lock step with them.
Israel is a pretty good ally from an intelligence standpoint. For instance stuxnet the virus that crippled the Iranian nuclear program was partially developed by Israeli intelligence.
Not just the military industrial complex either a lot of US companies have gone hard into the manufacturing sector in Israel. Intel has one of their high end fabs there and I see a lot of random shit in stores that was made in Israel these days.