If there were natives using or claiming the valley that wouldn't be something to brush off as unimportant. If the US government had a treaty or agreement with the natives recognizing that the land was theirs you would have to try to get a new agreement made taking the land out of the hands of the natives, which involved influencing the federal government to do what you wanted.
Furthermore, although many of the Indians in the west were non aggressive, even the non aggressive ones could react with extreme violence to efforts to take their land or otherwise abuse them. Thus trying to dispossess Indians could cause a conflict in which tens or hundreds of men, women, and children would be killed.
And sometimes Indians knew or believed that individual white persons were responsible for their troubles and sometimes they sought out the individuals they blamed for their troubles. So if you go to a lot of effort to have the government take land from a group of Indians, and if they learn you are a leader of that effort, it is possible that you might wind up suffering special attention from a group of angry Indians.
Trying to acquire land in the possession of Indians could be very troublesome.
As a general rule, the large ranches in the west were built up according to the saying that the rich get richer.
A richer rancher would have employees. And a rancher with employees could have each of his employees homestead an area that included a water source where cattle could drink. So the rancher would have his employees claim land along streams and around waterholes. And when the employees finished the homesteading process they would sell their homestead to their boss.
So a rich rancher would build up control of all the water in a large area. Other persons would be legally free to graze their cattle in the public grassland surrounding the big rancher's land claims, but to water their cattle they had to have his permission to use the water sources on his land. If they couldn't get permission to use his water they couldn't take advantage of their legal right to graze in the public grasslands around his claims. So a rich rancher could monopolize the grazing in an area of public grasslands many times as large as the lands that he owned.
And where there were periodic natural disasters or financial problems in the cattle industry, the poorer ranchers would probably go bankrupt and lose their land and cattle and the richer ranchers would be able to buy their lands and cattle and increase the sizes of their ranches.