I've never really delved deep into how they came up with "thousands of years"... I think you have prompted me to look into how those scientists came to their conclusions.
The reason why modern YEC scientists went with "thousands of years" is because they put their beliefs first ahead of their scientific endeavors. Their presuppositions start with their particular interpretation of their religious beliefs, then they try to use science to prove those beliefs true. When that fails, they try something else. Then, they try something else when that fails
While they continue to hit their head against a brick wall over and over and fulfill the true definition of insanity, the rest of the scientific body just keeps chugging along
That's not to say that religious people are bad at science - on the contrary. As far as I'm aware many if not most scientists happen to be very religious too. What separates them is that they do not try to prove their assumptions right in spite of the evidence. Instead, they simply follow where the evidence leads and go from there
If you are literally asking why young earth creationists chose thousands of years, I know from the Christian perspective it has a lot to do with things mentioned in the bible. Jesus' geneological line traced back to Adam, for one