Are some people just terrible at finding rhythm and can't get better at it? ?

I would hate to discourage you from playing the guitar, but I do feel that some people lack a sense of rhythm.  However, if you can clap your hands in synchronism with a song, or if you can dance, I would think that you do possess a sense of rhythm.  Perhaps you should try to develop your own natural strum pattern for a song, and not try to mimic a particular rhythm pattern.  Some rhythm patters are somewhat complex, and can be tricky.  If you're focused on a pattern like down, down, down, up, down it may be distracting.  Try to play along naturally with the song (on you tube for example) without focusing on a particular pattern.  Many people cover songs without adopting the original strum pattern.  Also, it may be helpful to study a little on music timing, for example, are you counting a 4/4 timing a 3/4 timing or a 6/8 timing.  The way you count time affects strumming. (most songs are 4/4 timing. ie 4 beats per measure)

I use to teach dance and yes there are people who don’t have rhythm. Who can not clap to a beat. 

My daughter now teaches ballet and tap. The schools districts in her area do not teach music of any kind in their schools. Budget cuts that eliminates music and art years ago. She says she can pick out student who have had music privately From those students who have not had music. 

If you don’t have a music background of any kind it may take you longer. Some people are just naturally talented, some are not.  

If you enjoy what you are doing then keep it up. It doesn’t matter if it takes you longer. Or perhaps you would benefit from having a guitar teacher.  

you might still get better, ask someone to teach you