It happened in my small lake recently. I was catching fish over 8 pounds until about 4 years ago, and it just stopped. The next 3 years only 2 that were just over 4. Here are my possible explanations.
1. Someone began catching and keeping them all of a sudden. I would say this is 100% not the reason. I do 90% of all the fishing done here. Only a few people bass fish, and they don’t go often. I know them personally, and we are friends and catch/release all the big females.
2. They somehow escaped through a drain or outflow. That is a small chance because our lake is lower than the outflows. Only during a heavy rain does it wash out instead of in. Even if some did wash out, not all , most , or many would.
3. A fish kill due to loss of oxygen or chemicals for weed spraying. We haven’t had one since 2011, which is a whole nother story.
4 Otters. This is my conclusion. I started seeing an otter occasionally starting a couple years ago. He hasn’t been back now in months, so I’m starting to catch bigger fish again.
That may be your problem, but on a big lake they have plenty of areas to hide. I’m betting they are still there but you just haven’t found them.