Those undergrads still need to be advised and they use lab resources.

If you're a tenure-track academic, your livelihood is much safer from having them try new ideas (that you will be the corresponding author on, increasing your prestige and ability to procure funding) instead of incrementing.

And if you already have tenure, maybe you have the undergrad do just that. But the tenure process heavily filters for ambitious researchers, so it's unlikely this would be a priority.

If instead you did it as coursework, you could get them to maybe reproduce the work, but if you only have the students for a semester, that's not enough time to write up the paper and make it through peer review (which can take months between iterations)