Wizards fill their skull to capacity with alien concepts so they can open short, somewhat-controlled gates to other worlds in order to make effects occur in their own worlds. This has consequences.
Within the first year of their death, a wizard’s skull bursts open and their rotting brain hits the ground with a acidic splat. Their spells become rooms (leading credence to the theory that all spells coincide with places, the subject of another treatise), their cunning becomes traps, their fears become monsters and their wisdom becomes treasure.
Also, their treasure becomes treasure.
Wizard tombs are designed like seeds, ready to expand when their arcane brains burst open with arcane pollen but it doesn’t matter. It becomes messy when a wizard dies and rots in an urban sewer, or a noble’s mausoleum or an abandoned barn only to have something grow in another place’s midst. Sometimes a wizard might die in a dungeon made from another dead wizard, leading to areas where their brains overlap and spells merge with spells, cunning with wisdom, treasure with fear. This is one way in which mega-dungeons are born.
A lich can take their own brain and seed it like a farmer, guiding the tunnels made from their own mind, pruning it like a tree.