If plants "alternate generations" between sporophytes and gametophytes, why don't we say the same of humans?
The big difference is that in humans, there is no mitosis in the haploid phase.

There are three terms that are important here:


Haplontic: Most of the life is spent in the haploid phase
Diplontic: Most of the life is spent in the diploid phase
Haplodiplontic (aka. diplohaplontic): About as much time is spent in the haploid phase than in the diploid phase


At the extreme, when there is absolutely no mitosis in one of the phase, then the organism is clearly not haplodiplontic. Haplodiplontic organisms have an alternation of generations.

Note that the term alternation of generations is also used for systems that are much more complex than haplodiplontie. Examples of such complex alternation of generation include many parasites such as the giant liver fluke