I agree that when playing solo, the past titles were way more fun. Up until BO2 and Ghosts for me. The primary reason for this is what they did with Merc mode. BO2 made it a moshpit, everyone left it. Merc mode doesn't even exist in Ghosts does it?
Merc TDM and Merc Dom were awesome to play as people could play and not worry about spawn trapping teams and organized groups. It was about fun and having 1 player turn the tide of the game. Yes, that can still happen, but not very often.
The removal of playable Merc modes is a core reason why solo play has basically become a less fun avenue of COD. I play solo quite a bit still, but it simply feels like grinding out games.