Or because not everyone agrees with your interpretation. By your definition all national parks are illegal since they are not included in Article I, Section 8. Land grant colleges are illegal and the Homestead Act was illegal since the government didn't own the land to give it away. The transcontinental railroad and the interstate highway system are illegal since they, too, aren't included in Article I, Section 10 and the government didn't own the property the railroads were built on.
You raise good points, all of those should revert to the States with feds retaining the authority to block toll charges and other things that would directly interfere with interstate commerce. The land grants built the states = red herring.