The problem with playing alone is that you can miss really simple things. For example, after analysing starting strategies at some length, in my second proper game playing axis, Russia hit MAN & UKR, and UK hit KWA, all on turn 1. This meant that the US plane in CHI, and the US fleet in HAW SZ could not be attacked.
Playing with another player will ensure that your mistakes are spotted.
Also, half the game is about second-guessing what your opponent is going to do - which way they will leap - whether they are going for the short easy kill or the longer war of attrition. In a game you have to consider all the possible attacks an opponent might make, and move your units to optimise your defence - when playing alone, it is far to easy to “predict” the other countries’ goals.