Sorry, I should be more specific. We could mean two separate things by "alternation of generations":

(a) Different chromosome numbers and different reproductive strategies for different parts of the organism's lifecycle. For instance, diploid (reproducing asexually) and haploid (reproducing sexually) forms in some plants.

(b) Different parts of the lifecycle look different, live in different environments, and eat different things (but are composed of the same kind of cells with the same chromosome count). This happens in animals a lot. For example insects invented metamorphosis possibly so worm-like larva and winged adults would not compete for the same food (also perhaps for dispersal?). Sea squirts invented a motile larva with a spiral chord to solve the dispersal problem. Some time later, highly specialized neotenous versions of that larva are walking around on the moon...

The fact that these kinds of lifecycles are "more complex" should not influence much our belief in how likely they are. Evolution doesn't care about Occam's razor at all, it will develop all sorts of crazy elaborate things if there are local reasons for it (and in fact frequently does so). Occam's razor is something of a kung fu style for scientists, not an organizing principle of the Universe.

I am mostly curious about the lack of (a) in animals. Generally by "alternation of generations" biologists mean (a).

There are some theories that dominant diploid forms arose because they are more resistant to mutations. OP sounded like a plant biologist (or someone in graduate school studying plant biology), so I thought I would ask an expert's opinion on this question.