Because you've never ridden one. Unless you're a total masochist or feeling suicidal, 5 minutes on a 1970 CB750 after riding a 2016 CB650 would be enough to rid you of any desire to go back in time.
In the seventies you also had shitty tyres like "Teflon Superslides" or whatever made by Bridgestone etc. They made the bikes feel even worse. For all their faults Jap Crap bikes were better than than the run of the mill Brit bikes though.
As someone without a driver's license with little/no desire to own a motorised vehicle.....
Whyyyyy do I want a classic (Japanese) motorbike every time I watch a 70's film?