If you look on the same map, there are a number of present day interstate highways that were not originally included on that map. A number of them have been added at various times over the years, since 1956, the passage of the original bill. One instance is Interstate 49 between Shreveport, LA and Lafayette, LA. That was not built until the 1980's and completed in the 1990's. Various federal Transportation bills through the years have frequently included expansion of the National Interstate and Defense Highway system. Through lobbying and political power plays, new interstate designations have and can be added to the system. The 1955 map, or other later versions are not the "be all and end all" of all interstate highways that will ever be built. Politics does play a role, whether it is for the good or bad of this country. Pork barrellism is the flaw of the system, and the National Interstate and Defense Highway System is not the result of some non-political, non-pork barrell, purely-for-the-good-of the country designation.
Here is a map of the National Highway System, High Priority Corridors, including the present day I-29 (placed on there because of NAFTA as the reasoning, but with 3,500 cars per day at the ND/SD border one may wonder about the need for the designation), and including a number of corridors that are not currently a part of the National Interstate and Defense Highway System, that could be someday:
Note: As with almost anything created through politics, this map could change in the future, and some road or corridor you wouldn't think should be included could through political influence and be justified through not only economic development and traffic needs but also through the National Defense needs. There may need to be some demonstration, but a like a lot of things, it could be arbitrary in determining it as a National Defense Highway if enough politics is played.
Another example is Interstate 22, which is almost built between Memphis, TN and Birmingham, AL. The road atlas doesn't show the I-22 designation, but the road markers are being changed as we speak. If you look at the 1955 map I included in the previous post, there was no interstate designated in 1955 between Memphis and Birmingham. It has been added in recent decades and is almost built and is now a part of the National Interstate and Defense Highway system, an amendment through politics, whether it be for the good or the bad.
Also, there is talk of a new Interstate 14 to run from Augusta, GA to Natchez, MS and eventually on into Austin, TX to provide an alternative east-west inner coastal South roadway because of the current damage and potential damage to Interstate 10 which is vulnerable to Hurricanes.
These routes were not a part of the original map that the Eisenhower Adminstration designated 50 years ago if not more. Things changes, human needs changes and the politics change.