Here's why:
* "Pretty" is an adjective, and "lot" is a noun. You can't directly combine them.
* "A lot" is a quantifier, meaning "a large amount or number." It's used as a single unit.
Here are some correct ways to express a similar meaning:
* "A pretty good lot": This emphasizes the quality of the group or collection.
* "Quite a lot": This focuses on the quantity.
* "A significant amount": This is more formal and precise.
* "Many": This is a simpler and more general way to express a large quantity.
The best choice depends on the specific context and the desired tone.