Clearance investigations are very extensive. They take months to years to adjudicate. They are cost and manpower intensive. There are times when the backlog of people waiting for clearance investigations to complete is months to years long.
I was going to bring that up too.
My very first SBI in 1965 took several months. I was only 18 and had zero record. Not even a traffic ticket. They even investigated those people I used as references.
I'd heard those cost the government on average $10,000 to do.
That's much of the reason they're only done every five years. Time and money.
I maintained a TS with SCI access for over 30 years. Every five years well before my clearance would expire I had to submit a new DD-398, and finger prints.