Why do presidential candidates make promises that they won't be (much) a part of?
It's one way for the people to tell Congress what they consider to be important.
Article II, Section 3,
He shall from time to time give to the Congress Information on the State of the Union, and recommend to their Consideration such Measures as he shall judge necessary and expedient; ....
If a president feels that a program, such as Medicare For All, is necessary and expedient, then such a recommendation falls within the obligation of the presidency. One way for the people to decide the direction that should be taken by Congress, is to elect the president based on such campaign promises and provide that president with a large mandate (much more than 50% of the popular vote).
Because the president is the de facto leader of the political party, it then falls upon the party members in Congress to help the president fulfill those campaign promises.
Of course, politics in the U.S. is often about not doing with the other person promised.