There is an interesting 6-part LTSpice overview/training on YouTube by none other than the creator of LTSpice itself (Mike Engelhardt) and at the very start he mentions why he thinks that the best simulators are written by the concerns that need them (e.g. Linear Technology at the time, Analog Devices now) as opposed to software companies. See https://m.youtube.com/watch?v=6gKThjZIj-s.