Because nobody wants a 4.21567Kohm resistor, not enough
to be worth all the special handling. Mass production to a
target, or a manageable number them, is where it's at.
Absurd we have a particular resistor for every purpose and to suit that we can't use an arbitrary resistance but we go specific resistance how to go for specific ones with out any specifications
The use of reduced range series implies in a reduced BOM itens variety, reducing supply cost.
Other advantage - regarding SMD assembly - is reduction of reel exchange performing work with less steps.