It's been a long time since I lost to the AI without bullcrap like "vassal attached army detaches right before battle" or "general dies marching to combat and you lose 6+ pips", excepting cases of trying to break out as an OPM and screwing up.
When I was practicing scorched earth and unit control in 1.4 (I've only had this game since December 24), I learned from Stategem and then played two starts until I could win them:
1. Take Mongol Khanate, and consistently defeat Oirate Horde 1 vs 1 in your independence war. Note that they start with over 2x your forces and a shock V general. Oh, and no loans needed.
2. Pick Ethiopia, fabricate a claim on the Mamluks at the 1444 bookmark, and then declare immediately once you've finished the fabrication. Win the war and take at least one province without any allies. Loans optional (I didn't take any).
If you practice stuff like that and know the game's mechanics really well, fighting the AI at similar tech/numbers just isn't the same any more. Hitting the timing and completing "Luck of the Irish" before 1530 was much easier than beating the Mamluks with Ethiopia for example. You also eventually realize what works and what you *can't* get away with, like beating Spain solo as Grenada at the start, because SE doesn't do enough damage there.
Just recently I declared on Venice as Byz in early 1500's, pulling in France which was more than I bargained for (normally, they could not reach me, but got involved vs Austria as well and marched through their Austrian foe to reach me :/), and still came away with a province returned to me due to stack manipulation and retreat-attack wipes. I screwed up and lost some naval units there before retreating them and had 60 units in territory from a better tech group vs my 30, and took a province

. AI is *not* the limiting factor in this game unless you botch something. Admittedly, I'm not perfect and I occasionally botch diplomacy or unit control, but most of the time the bottleneck isn't there and it isn't close to there.
It's more prevalent and noticeable in MP, because it results in losing outright instead of just significantly and unnecessarily slowed progress for no other reason than "because". However, it is not MP-only.
Even if it were, MP is a real part of the game and the game needs to work in both SP and MP.