Because the Egyptians didn't keep slaves. Their soldiers took slaves as booty, and sold them or kept them on the road, as they traveled and conquered. They also enslaved foreign miners to exploit their resources. And they used foreign domestic servants that were not necessarily "slaves". But they didn't use slaves to build their pyramids and temples because that was considered a sacred quest by and for the Egyptians, themselves. They were earning their passage into the next world. They would work the fields part of the year, and build monuments the other part. Because the Nile valley was so fertile they could do that.
Interesting, that might be why the Israelites were forced to build a store city and not a pyramid or temple.
And I don't believe it because the rest of the story is unbelievable. It is a mythical story that neither requires nor purports to be historical fact. It's intent is to convey an ideal, or an ideology. Not to tell us what actually happened.
It conveys the idea that YHWH is greater than the gods of Egypt and it (along with the stories of Abraham, Isaac and Jacob) sets the stage for why YHWH is the King and God of Israel and their redeemer,,,,,,,,,,,, and why the land of Canaan belonged to Israel etc. etc
To me if it did not happen then the whole thing is no more than a fabrication and lie. And I guess that is how you feel about it also.
The interesting thing is that it looks like the remains of non slave Hebrews have been found at Goshen (where the story says Israel stayed while in Egypt) and at the right time (given the Biblical dating for the Exodus) And given that dating the conquest remains in Canaan agree with the story of the conquest in Joshua.
Of course there is the thing about the story being unbelievable because of the miracles, the usual naturalistic way of reading such stories.
As with most myths, some of it is probably grounded in some actual events. But that is not the purpose or significance of the story. So for me, the fact that those people were not enslaved in Egypt is irrelevant to the value and meaning of the story.
If it is not true does the story have any value and meaning apart from being an origins myth? just as all the other supernatural miracles and prophecies etc in the OT are explained away in the same manner. (they are unbelievable) Till the OT becomes nothing but a big pack of lies when it comes to anything serious about God,,,,,,,,,,,, and the NT is dealt with in a similar manner.
But that is the way many people believe and see it.
To believe there is a possibility for a creator God to exist then people who say that they would believe if there was evidence, should really say that the OT and NT stories could possibly be true and not that they are unbelievable-------------thereby eliminating and denying the actual evidence.