There no law that says a husband and wife have to be buried next to each other. In fact, I don’t always assume that both members of a couple were laid to rest in the same cemetery.
They could have died in separate locations decades apart and taking the one who died last “back” for a burial may not have been feasible.
The surviving spouse in the couple may have married again and been buried with the subsequent spouse. One spouse could have been married to a previous spouse with whom they are buried.
The couple may have eventually divorced or simply separated and, for obvious reasons were not buried together.
The spouses may be buried next to each other but, for some reason, there is no record for one of them indicating where they are buried.
There may be a separate reason that I forgot.
Try a Genealogy Search on GenealogyBank.