My theory is that brambles are carnivorous

. They send out those long, thorny canes which wrap around the legs of passing sheep and humans, and hang on to them tightly so they can never escape. Once dead and decomposing, the brambles feed on the nutrients released by the corpses
If you have dense thickets then pulling them out by hand won't work. You really need to cut off the long canes back to just above ground level and have a huge bonfire. I use big loppers and wear strong gloves and Barbour jacket and trousers, with sturdy boots to get in amongst them, then cut off sections as I can reach them until I am down to the stumps. Once all the tops are cleared away then you can drag or dig out the roots. If it's a large thicket then it's a big job.
I have heard of pigs clearing brambles but I've no experience of it.
The wild brambles around us are not tasty so I grow cultivated varieties. Some cultivated brambles don't taste all that good, but a few are really delicious. Unfortunately it's the thorny varieties which taste the best, so you have to keep them well controlled and trained over wires or they reel you in just like the wild ones.