I think previous care determines the stem length.
A properly prepared Amaryllis yes, is very easy to plant and bloom. One can well imagine that discount bubs are not all well prepared- or maybe have suffered in transit and store display, not to blame the grower.
Will you keep them indoors or can you plant outside when spring arrives?
Amaryllis need plenty of sun and fertilizer over summer, to rebuild the big bub for more blooms. Indoors, just keep in as bright as possible light and water as any houseplant. The big leaves may be floppy though, they aren't a great indoor plant.
