Monster (Lich et al) Stat Blocks

Why do Lich's and other equivalent monsters that have their origins in being a being similar to a player character (presumably a wizard) not have spell casting abilities that resemble such.  I would have thought that a Lich would have spell casting abilities that resemble a wizards (with possible enhancements) in terms of spells, flexibility and the ability to learn more rather than the limited forms of spell casting that we are presented with.  Otherwise, why become a Lich?

Monster stat blocks are designed for a fight. A single fight at that. Once the initiative starts, the monster’s life expectancy drops to less than 1 minute. As such, the stat blocks are made to show an effective load out for that CR creature. The designers tend to cut out the utility spells they might have ready, but won’t come up in a fight. Like, a lich who can scry makes tons of sense, but a lich who cast scry in battle would be very unusual. So they leave it out so as not to clutter the stat blocks.

If you as the DM want to give a lich a pack of other spells, have at it. It really makes sense that they might have a bunch of other spells prepped.