Criminal Law/License Not required in US for noncommercial traveling
Not entirely sure if this is the best place for this, but it seems best suited. Recently I've been doing some reading online, and what I've gathered through it is that if you are a private citizen traveling for noncommercial reasons, then technically you are not required by law to have a driver's license or registration because of your right to travel.
U.S Supreme Court, Reno v. Condon - January 12, 2000; The activity licensed by state DMVs and in connection with which individuals must submit personal information to the DMV (the operation of motor vehicles) is itself integrally related to interstate commerce.
"Personal liberty largely consists of the Right of locomotion -- to go where and when one pleases -- only so far restrained as the Rights of others may make it necessary for the welfare of all other citizens. The Right of the Citizen to travel upon the public highways and to transport his property thereon, by horsedrawn carriage, wagon, or automobile, is not a mere privilege which may be permitted or prohibited at will, but the common Right which he has under his Right to life, liberty, and the pursuit of happiness. Under this Constitutional guarantee one may, therefore, under normal conditions, travel at his inclination along the public highways or in public places, and while conducting himself in an orderly and decent manner, neither interfering with nor disturbing another's Rights, he will be protected, not only in his person, but in his safe conduct." [emphasis added] II Am.Jur. (1st) Constitutional Law, Sect.329, p.1135.
Your research has not let you down. If you are walking on the side of the roadways you should be fine. However, when driving a vehicle a State can compel a citizen to be legally licensed and to have their respective vehicle registered, inspected, and insured. That is for the protection of all citizens sharing the road. The Full Faith and Credit Act allows the respective States to recognize driver's license, insurance, and registration standards.
By having standards the respective States can ensure that only people who can operate a vehicle safely are doing so. Under the same logic the vehicle itself must be safe to operate so that it is not leaking fuel, ready to blow parts off on the highway, etc. In like kind the need for the vehicle to be insured will/does protect the other motorists should an accidental collision occur.