I’m assuming, you are referring to a HDD and not a SSD?
On the other hand, the reason in both cases is, that usually over time the amount of data on the drive increases. Therefore it takes longer to find what you are looking for, out of the total amount of data.
With SSDs that’s faster, because it’s electronics and chips and allows random access of memory addresses.
With HDDs that’s a bit different. HDDs are mechanical and magnetical drives. They don’t allow random access. They are sequentially read. So, the more data you add, the slower the whole process becomes. And given they are already way slower than SSDs and usually offer more storage space too, you usually feel that a lot more than with your SSD!
Adding to that are cluttered operating systems and generally slower processing of data. So, stuff like blooming temp directories and tons of temp files and all that stuff and poor system management and tons of background processes add additionally to slowing everything down.
So, although the technology and hard drive are actually not slowing down, your device doesn’t just become seemingly slower. It actually takes more time to process all the data.
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