This year, so far, I've read 19 books
I could claim lack of time as a reason for reading so comparatively little, but that's a bit of a cop out. Yes, I have a full-time job and several commitments outside of work, but I am guessing those who read 100 books plus a year have various commitments that they have to juggle as well. The reality is - like with anything in life - if you truly want to do something, you try to find ways of doing it. Therefore if I wanted to read more books, I would find ways of reading more. In fact, there are plenty of times when I could be reading but I choose not to.
For example, unless I am particularly engrossed in a book, I tend to watch telly than read in the evenings. After a long day, I just want to disengage my brain and watch something a bit mindless. I find the prospect of attempting to understand a plot or characters too taxing. That said, I do always read a few pages of whatever book I'm reading before going to sleep; I just rarely - if ever - spend a whole evening reading.
My daily commute to work - one hour each way (the joys of living in London, hey) - probably provides the greatest opportunity for reading, but actually I tend not to get that much done. Well, it is probably more accurate to say I don't get much book reading done as I tend to read other things when travelling to and from work. I read a newspaper in the mornings - unless the news is too scary and likely to trigger my anxiety (which, funnily enough, has been happening quite a bit of late) - and often read a magazine on the way home. Mind you, during rush hour, reading anything at all can be physically impossible at times. When you've squeezed yourself onto a Tube train, reading a book/paper with one hand while you cling onto a pole with the other is a bit of an art form - particularly if you're rammed up against other commuters and can barely move.
Another opportunity to read more would be to listen to audiobooks. I used to be quite snobby about the idea that audiobooks counted as reading, but I am now beginning to realise that they provide just a different way of "reading". For some people of course, such as those with visual impairments, audiobooks are the only form of reading available to them. Therefore to say that listening to audiobooks is not reading is to be dismissive of how whole sections of society experience books. I have, in fairness, considered listening to audiobooks but I find the price prohibitive. On Audible, they can be as much as £20 each and I don't think I would use the service often enough to warrant paying for a monthly subscription (about £8 per month). So for the time being, at least, I am content to download the odd narrated book off the BBC iPlayer Radio app (wouldn't be accurate to call them audiobooks given that they're often abridged).
Sometimes I not only ignore opportunities to read more books, I also wonder if I should read fewer books (which surely must amount to sacrilege for a book blogger!). I've read 19 books this year and, to be honest, I'd struggle to remember what they all were without checking my Goodreads list. I don't think this is a reflection of a quality of the books (though some, admittedly, were quite forgettable) but more that, as a human, I can only recall seven thing at any one time. Does seems a tad pointless to read more books if I am only to forget that I've read them afterwards! (Ignoring the whole counter argument that you don't need to remember a book for it have a lasting impression, obviously.)
Ultimately, of course, reading is not a competition and there are no rules about how many books you should read in a month. The amount of books you get through in a month is entirely down to you.