Practically speaking, they're lumped together for a few reasons:
1. Many people who like one genre also like the other.
2. Many authors write in both genres.
3. There's a lot of similarity in the genres, and lots of things that are hard to categorize. More true lately btw.
Just as an aside though, I personally was an avid almost-only-SF reader for the first 30-ish years of my life, but lately have been reading a lot of fantasy as well. I highly recommend trying, especially more modern fantasy - I feel like the lines are even blurrier between them today, and a lot of the best work today has shifted from SF to fantasy. (I still love SF and there's a lot of great SF as well, to be clear.)
1. Many people who like one genre also like the other.
2. Many authors write in both genres.
3. There's a lot of similarity in the genres, and lots of things that are hard to categorize. More true lately btw.
Just as an aside though, I personally was an avid almost-only-SF reader for the first 30-ish years of my life, but lately have been reading a lot of fantasy as well. I highly recommend trying, especially more modern fantasy - I feel like the lines are even blurrier between them today, and a lot of the best work today has shifted from SF to fantasy. (I still love SF and there's a lot of great SF as well, to be clear.)