It's not like the Japanese Prime Minister (and former Prime Ministers) don't have security, but the disparity of security compared to US Secret Service protected individuals is huge.
The fact that the killer managed to get so close to Abe without being screened is indicative of the fact that such an attack was so unexpected in Japan.
True, I'm just saying that if someone wants to assassinate a current or former American President, that would be a massively more difficult hit.
The Abe killer was unscreened (like everyone else in attendance), and managed to walk directly up to Abe from behind and fire shots from 7, then only 5 metres away.