It looks easy on You Tube when they back the bike up, but I have not been able to pick up my bikes (GS500, SV650, DRZ400sm) by backing my butt into the seat like they do. Usually my bike would be lying a weird way, on dirt shoulder, or a slope, with loose footing, and I just couldn’t seem to squat that low and get enough foot traction and body leverage to back my body into the bike. It could be the type of bike and how low it lies, like eon said, and it could also be something wrong with my technique.
What I do instead is to use the handlebar as a lever, squat down and hold the grip with both hands, and dead-lift the bike up. Since this is a straight-up lift, loose footing is much less of a problem (except the time when I was standig on a steeply sloped dirt trail). You need to have enough strength to dead lift 70-90 lbs (my estimate) initially with the bike lying flat, depending the bike, but it quickly gets lighter as the bike moves upright. Still, I got a sore back for days every time I did this. Again, poor lifting technique.