Chess is purely tactical. It is all about the moves. Also you only have one opponent and no help in chess when you get into a bad position.
Diplomacy, on the other hand, is actually less about tactics than communication. When you are in a bad position, unless the entire board is equally out to kill you (possible, but unlikely) you usually have a chance to turn things around by pitting other players against the person/people trying to take you out. I've come back from three centers before to be part of a three-way draw. I was England and France and Germany were pounding me, until I convinced Russia and Italy to join me in a three way alliance. Turkey was busy with Austria and vice versa which left Italy free to crawl up France's backside and Russia was fairly stalemated but I offered to hand him Norway if he would send forces west. That only left me the three centers in England.
Soon France and Germany found themselves in a two-front war. They couldn't focus on taking me out but had to defend their eastern fronts which allowed me to grab a foothold in the lowlands and force my way into the English Channel. I was talking my behind off and coordinating moves between Russia/Italy and myself. They were getting gains so were content to let me play "field marshal". It worked brilliantly and Turkey crushed Austria right about the time we had Germany and France on the ropes. I convinced my "partners" to turn their attention and cooperate on Turkey while I mopped up the remnants of F/G.
Many players, faced with F/G coming at them and beaten back to their island with French and German fleets in the water might have surrendered that as a "hopeless" position. But remember, you are never alone in Diplomacy as long as you can convince someone else that it is in THEIR best interest to help you.
That's why I agree with the above poster who cringes at the comparison of Diplomacy to 7 person chess. Chess is still about tactics. Diplomacy is about strategy.
All that aside, it also really does unbalance the game if you have several centers and surrender. Whichever country in in the best position to overrun your zombie forces has a decided advantage in the rest of the game. Actually, I have strengthened my position and gotten TWO opponents to back off me just by threatening to "kingmake" the other. If two nations both think you will fight them tooth an nail while handing your SCs over to one of their opponents, it can be a way to buy some breathing room while you figure out a way to turn things around.
The options are manifold to turn a bad situation around, but if you surrender, you eliminate those possibilities. Honestly, for me, turning a bad situation into a share of a draw is a much more satisfying result than bulldozing a solo.