I'll treat this question as simply as I can. Col, since you don't know the details and implications of what you ask, here are some key points.
Does the most powerful man have the power to ban the NRA and make being a member illegal?
No. The USA constitution also guarantees Freedom of Association, which in the past has been taken to include the right to join an organization as long as that organization doesn't advocate sedition or insurrection. Further, if the organization were to be accused in that way, its members would have the right to challenge that accusation in court.
The NRA has not, insofar as I remember, advocated sedition or insurrection. They advocate gun rights and gun training. They have the publicly avowed intention of preserving gun rights under the 2nd Amendment to the constitution, which is, at least
prima facie, in support of a guaranteed right of the people, which therefore is NOT within any of the definitions that would make the NRA illegal. If they have a hidden secondary agenda, it remains hidden.
why doesn't the president or the government change the constitution to make gun owning illegal.
Because neither the President nor Congress nor even the USA courts can do that. Changing the constitution is a right expressly granted to the
people through the mechanism of formally amending the constitution, where 38 states (actually, 3/4 of the states) must accept the proposed change. The process is called "ratifying an amendment" and is described in the main body of the USA constitution. At the moment, I do not believe enough agreement exists among the general population to support that particular action. Among many states with hunting traditions, that would be a non-starter.
Americans accept it rather than demand change.
Americans
are demanding change but the politicians are, as usual, pressing their own tunnel-vision agenda in a way that will not have the desired effect - and the majority of the people know that. Some so-called "red flag" laws might get passed, laws that would allow police intervention if someone appears to represent a gun threat.
The REAL problem is that folks have not been, and still are not, willing to "blow the whistle" on someone else. The Uvalde shooter posted stuff online that, if it had been forwarded to police for their attention, might have prevented the incident entirely. We used to have an old phrase, "to drop a dime" on someone - meaning use a pay phone to call the police. Now of course we have so many cell phones that the phrase no longer has a meaningful referent. But it still remains the best solution. Sadly, too many people are so self-centered that they don't want to get involved and thus let pass those opportunities to prevent such incidents.
EDITED by The_Doc_Man to fix a couple of awkward sentences and to correct one more statement. The number of states required to ratify a new amendment is 3/4 of the states. That was true. But 3/4 of the states, or 75% would be 37 1/2 states and they round DOWN. So I originally said 37 but actually, we need 38 states (=76%) to ratify any new amendments.
@ColinEssex - I hope this sufficiently answers your questions.