From previous meetings, Real Madrid has always been beating PSG when they meet, especially in their last games where PSG was ahead in the first leg but failed to qualify because they lost the return leg. I believe the reason why the odds are like this now is because, PSG is the best team in Europe. I also believe that Real Madrid having almost similar odds here against PSG is because they have improved this season compared to last season of which the odds would have been higher only if it was last season.
"Always" is a bit of a stretch.
The 6 most recent head-to-head results between Real Madrid and PSG are:
Mar 09, 2022: Real Madrid
3-1 Paris Saint-Germain (Champions League Last 16 Second Leg)
Feb 15, 2022: Paris Saint-Germain
1-0 Real Madrid (Champions League Last 16 First Leg)
Nov 26, 2019: Real Madrid
2-2 Paris Saint-Germain (Champions League Group Stage)
Sep 18, 2019: Paris Saint-Germain
3-0 Real Madrid (Champions League Group Stage)
Mar 06, 2018: Paris Saint-Germain
1-2 Real Madrid (Champions League Last 16 Second Leg)
Feb 14, 2018: Real Madrid
3-1 Paris Saint-Germain (Champions League Last 16 First Leg)
So we had Real winning 3 times, one draw, and PSG winning twice.
I wouldn't be putting too much weight on it, as any club is only as good as its recent performance. The last clash between these two was in 2022, and a lot has changed since then. That's why it's the PSG, not Real, who are a (slight) favourite.