Why are time signatures so fxcking confusing?

I have never seen a 7th note or a 9th note but I suppose it could exist. 

Theory teacher here.  The bottom number represents a NOTE VALUE- 4, you are counting quarter notes. 2 - half notes.  8 - eighth notes.  You can have AS MANY AS YOU WANT in a measure, to fit the rhythm you are writing.  13/8.  3/4.  That upper number tells you how many beats. Sometimes we break it down further, sometimes

 we consolidate - but it MUST add up in every measure.

 Do you ever pay in cash?   If you owed me 3 bucks, you could pay all in one dollar bills, all in quarters, all pennies, all nickels - as long as it ads up.  That is you meter signature (time signature).  In real life, you would pay with any combo of money you have - as long as it ads us to that 3 bucks.  There are some time signatures used a lot - and others less often - and some, almost never.  I had Bulgarian jazz album in which the pianist counts it off for the band - because it is in something like 21/16 meter! Years ago, I lost the album that had that track on it, with him playing to the band while yelling out the beats.  I tried to get a CD of that - but it only had the band playing, after hours of rehearsal.  The counting track was hilarious.  Milcho Leviev.  Don Ellis band.  You need to calm down and learn your skills.  They is no way to skip ANYTHING if you want to be a good musician.  I have been a pro full-time since 1973.  Still practice and learn new things every day.  My phone continue to ring with offers, new students, concert bookings (for the future . . .) and gigs. Not a coincidence . . .

Where have you come across 17/7 or 15/9?

the only people who play in signatures like 17/7 or 15/9 are math rock bands doing it to be deliberately confusing. Functionally, there really aren't that many time signatures you need to know.