You're right. It's not the girls fault at all. But sometimes people just won't take no for an answer so it's safer to say you "belong to another man" (Not how I view it but how others do at times) than to say you just don't want to be with them
Plus when you say no, They push you for reasons why. Or they beg you to give them a chance. And generally just pressure you until your no turns into a yes. Something I have experience with. Saying you have a boyfriend, while not always perfect, generally shuts down the conversation quickly. You're right that girls shouldn't lie about but sometimes it's just safer and easier.
...and that's still nicer than telling something bad about his personnality. Seriously I've rejected guys saying they weren't my type/I wasn't physically attracted to them and there was no hard feelings, but one of them was so convinced he was Apollo himself that I had to tell him I simply didn't like who he was as a person, he asked for details and to this day I'm still haunted by the look on his face.
The look of a dying man, someone who was losing all confidence he ever had in himself. I was sorry as it felt like I was debuilding him piece by piece but he wouldn't take no or the look stuff.
Just be upfront and tell him you’re not interested.