I trust your comment was intended sarcasm.
The great plains could not grow forests for the bison would graze off the saplings that did spring up. There is no new tree growth in natural wooded areas cleared for cattle grazing.
(Pay particular attention to the part about the Indians.)
The real story, or so it now seems, emerged piecemeal over a century and may rightly be regarded as one of the triumphs of the science of ecology. The question was squarely framed and partly answered in a classic 1935 paper entitled “The Prairie Peninsula,” by botanist Edgar Transeau. Numerous others have made important contributions since, as summarized in a 2003 paper by weather scientists Stanley Changnon, Ken Kunkel, and Derek Winstanley. The chief factors:
Drought. Notwithstanding relatively plentiful average rainfall, the prairie peninsula suffers from severe drought 50 to 200 percent more often than the surrounding forests.
Dry season. In contrast to forest regions, which have relatively uniform precipitation throughout the year, the prairie peninsula is noticeably drier in late fall and winter.
High ratio of evaporation to precipitation. A key insight of Transeau’s, this one gets a little technical, but the main idea is that despite abundant rain, plants dry out faster in the prairie peninsula due to wind, temperature, and so on.
Flat terrain. The prairie offered few natural barriers and particularly–you see where I’m going with this–few natural firebreaks.
Lightning. After Florida and the Gulf Coast, the prairie peninsula has electrical storms more often than any other region in the U.S.
Fire. There seems little question that recurring fire promoted by periodic dry spells was the central formative feature of the prairie. How the majority of fires got started remains a matter of debate. Native Americans evidently torched the prairie frequently to create more desirable grazing land for game. Other blazes were started by lightning, which often struck the highest thing around, namely the trees. Whatever their cause, the fires were certainly dramatic, racing across the prairie at speeds of up to 15 to 20 kilometers per hour and incinerating vast tracts. Forests were slow to recover from the destruction, but prairie grasses, whose seeds and buds remained cool a few inches below the scorched surface, were back the next year. Grasses, in short, thrived because they were better adapted to the stressful prairie environment than trees, surviving everything except civilization’s appetite for arable land.
Cecil Adams
https://www.straightdope.com/columns/read/2604/why-dont-trees-grow-on-the-great-plains/