That's just is. 1,000 is not a specific number. It occurs often in Scripture, but rarely outside of symbolic use.
Temporal and Eternal Existence Compared
Unlike Jehovah's Witnesses and Adventists, theists do not believe that God exists within the realms of time and space. Time is believed to be a facet of the universe created by God. Theists call this "temporal" existence. If I understand them correctly, their belief is that time is our means of measuring change experienced on the temporal plane.
A peculiar aspect of this belief is that it shares a practically identical understanding of time as held by physicists, including the belief that time is relative.
God, according to what these people teach, lives in the "eternal." The eternal is definied by God's existence (and, according to this belief system, everything else is as well). To them, God "is." We might speak in such terms. such as saying God "was" and that God "will be," but those are understood as relative statements from the temporal perspective. Since God does not change, there is no was or will be with God. God is, and just that, "is." God transcends time to these persons.
Oral Transmission of the Number 1,000
This belief is ancient and goes back to the oral teachings of the Hebrews that helped shaped the written text. It was carried over into Christianity and is even shared by Islam. It was with the invention of dispensationalism that Adventists abandoned the view that God existed outside of time. If we were under the constraints of time, so was God, they taught. This teaching was absorbed by the Witnesses through Russell who got it from association with Barber and the other Second Adventists.
So for Witnesses and those dispensationalists who hold God as subject to time the same way humans are, expressions in the Scripture texts are literal. To this theology 1,000 years is 1,000 years (but there are arguments as to whether these are solar or lunar years or a mean between both). While recognizing the context of Psalm 90 and 2 Peter 3 as symbolic, this theology abandons the rule for both uses of 1,000.
Agreement Between the Two--Where?
It should also be noted that both theologies are in agreement that in all other uses of 1,000 outside of a connection with time, the terminology is secondary to accuracy, always carrying the meaning of "many" or "much" but with no specific value. That is why some census counts, notably in Samuel and Kings, are always rounded up and often look unrealistic (counts of people and army members are always perfect, i.e., 245,000 and 50,00, etc., never 245,678 or 51,235). In all these instances the number 1,000 is a narrative device meant to keep the reader from paying undue attention to details that might interfere with the subject at hand.
Again this rule is taken as universal for all its uses in Scripture except in the eyes of Adventist groups like the JWs who hold God as being subject to time like humans, thus evaluating all numbers with some specific value.