Please keep in mind that if you are casual, I assume you are not playing ranked.
Second - even this crazy bad matchmaking rarely puts silver players into Diamond+ lobbies, I have seen a few games where Gold players get into same lobby as current Master / Preds but that's about it while being on board of opinion that Preds/Masters shouldn't face even low level Diamond or below even more so = skill gap is just too big.
Finally it is known that many people by boosts, profiles, exploit game to gain badges, many seasons (stopped counting at this point) had free to take Masters (you could rat to Pred even)...so I would say you are not facing actual Masters rather players that was free to take masters / rocking some badges people pay attention to but reality is you are like Silver or below and they are like low level Plat that still feels like mission impossible to you.
You can post some screenshots of those Masters you are dying to and I will gladly decode them so you don't panic too much.
All I can say I am hardstuck Diamond but I have seen close to 0 legit Masters that I can't compete vs. Legit Masters I can take 50/50 on any day of the week. Problem is most Masters hack and you can't beat software.