I think not all sports gambling can be consider as fixed. I am playing sports gambling for a long long time. even my more interest is in sports gambling, for most of the time I found sports gambling very fair and even make a lot of money from it. Therefore I do not think that sports gambling are always fixed.
Same as me, till now i have not faced such an issue like games being fixed.
I would just called them rather rigid.
What is generally important is to utilize gambling platforms with very good reputation for those who may want to gamble. There are some gambling platforms that are bad eggs and they are just there to fix their site in their favor even if they claim it is provably fair and those are the scam ones. However, for reputable ones, there is what the call provably fair with the usage of some mathematical algorithm to control the bets.
Well, you could test the provably fair of different sites. Try to read this one
The easy way is just to check if there are a lot of players playing and continues to, then that's the best way to see if it's fair.