Why are agency MBS Spreads Tightening for Fannie, Freddie and Ginnie?

I noticed that the spread on Fannie Mae, Freddie Mac and Ginnie Mae issue pools are tightening to almost the lowest level for about 10 years.

I know you can't plot a spread like you would on a normal yield curve because you have to allow for the prepayments, and you can't get a very distant point on the curve.

But let's say you take a yield at a given point on the Agency MBS curve, adjusted for an expected prepayment pattern, and compare it to the same point on the treasury, that spread has been narrowing a lot.

Source: https://atlantisdatasolutions.com/agency-mbs-yield-curve