The fed has calculated that the seignorage vs. printing costs of switching to coins about balance out for dollars.
Pennies however: we keep them just for nostalgia.
[0] The $1 notes also go through a lot more use in their life so they degrade much quicker iirc and a bit over 40% of the bills in circulation are $1 bills too so switching over to a coin that lasts decades instead of 6-7 years (https://www.federalreserve.gov/faqs/how-long-is-the-life-spa...) would really cut down on printing costs.
Give it 15-40 years and we'll think about it again!
Of course, by then, so much of commerce will be electronic that I don't think anyone will care about what notes still circulate, other than the $100, which is used more outside the US than inside.