Because they can get away with it, its generally brtushed off (if acknowledged at all) as just being 'laddish behaviour' and still blamed on the woman who should have noticed she was getting drunk too quickly or should have had one of those testing stick things or whatever, if she did not, then its clearly her fault. Twas ever thus.
I have been spiked by drugs. I think it was royhypnol but I was never tested so cannot be sure. Luckily though, I was actually out with my partner at the time, though we were separated for an hour or so so the guy obviously thought I was alone. By the time the drugs kicked in, my partner was back and took me home. It was a very scary experience though all the same and made me realize just quite how much some men just do not give a tiny crap about what women want, or their health, or anything at all besides their knobs.
Another time I had the extra alcohol thing. I do not accept drinks from strangers, however I was with a male (ex)friend and we were taking turns getting the rounds. I think it must have been when I started getting a bit tipsy that he started topping up the alcohol. When I confronted him about it he said he had been buying triples (!) in an attempt to get me to lighten up a bit as I seemed down. Yeah, nothing to do with him trying it on at the end of the night eh?! Again, luckily I was kind of in control of myself, as I had noticed myself getting a lot drunker a lot faster and had moved onto just cokes, at which stage he said he would walk me home..probably because he realized that this was the only chance he would get as not drinking anymore alcohol would mean I start getting more sober, not drunker as was the plan. I have never since let anyone buy drinks for me, except for my parents/siblings and my (now) husband. Its too risky. But yeah, there are many other ways to be caught out by these pervs and its never the victims fault that someone treats them in this way, regardless of what they do/don't do.