I always wondered why they do that. Why not release their decisions fairly promptly after they are made and all of the opinions are written?

The opinions are still being written all the way to the end of the term. It's not like they write them and let them just....age.

Usually the unanimous opinions and the "simple" opinions (opinions where there is disagreement among the justices on the outcome, but where the legal cases don't necessarily have a whole lot of moving parts) get decided quickly and written first. Those opinions are either short, or easily written by the law clerks without a whole lot of justice input. They also tend not to have too many Justices writing their own separate concurrences or dissents. They take less time, so they get done quickly.

The other ones - including most of the major cases - take a lot more time to write. They might involve more issues, and often more research. They might require the Justice writing the opinion to have to work pretty hard to get other Justices to sign off on all their reasoning - so those are the opinions that involve many multiple drafts being shuttled from Justice to Justice to work on language and argumentation. There's often a ton of internal negotiation over those really contested and important cases, as the assigned Justice tries to hold together their majority coalition and keep the other Justices from bailing on the majority opinion and writing a concurrence instead. It can take a long while to get the opinion finished because of it.

Plus, there's still the rest of the Court's business. They're still preparing for and hearing oral arguments on new cases (arguments continue through April), they have their usual emergency docket, and they're reviewing petitions for cert for next term throughout this term. They're not spending all their time writing opinions.