Yes, it is hard to fire or remove a special counsel. There are only certain reasons it can be done, so they can survive changes of administration, which an AG wouldn't. John Durham, appointed under Trump continued under Biden and brought charges, although it all essentially fizzled out.
It may have to do with manoeuvring to make it harder for the Republicans who will shortly be controlling congress to interfere/derail the investigations. Certainly, they're planning to tie the justice department up in endless hearings and investigations (even threatening to impeach Garland) as soon as they can and this move places the investigation out of reach of that.
It may be because they're moving close to indictments and Garland believes (because he still lives in the world of doing things properly) that this could help de-politicise the situation. I also read someone theorising that it might be because it gives them scope to expand the investigations into some sitting members of congress.
Garland certainly seems unswerving in doing things the right way, so I really hope this is a good move.
NYT Special Counsel explainer