Possibly because you are doing hard work to climb, and doing very little work descending.
if the descent is long or steep enough, you might not even be pedaling.
But if you don't pedal, you don't climb.
Obviously if you had a 400-lb bike you would have a lot more momentum on the downhill. Of course, it would take a pretty long hill to get up to speed, but it would overcome as Lot of wind resistance with inertia.
Of course, you probably wouldn't be able to ride the thing on the flats, hardly, and forget climbing.
Also ... no matter how fast you go on the downhill ... the next climb is just as steep and hard. You might climb at 10 mph for an hour, zip down the other side at 50 mph and get there in 12 minutes .... and have another hour climb at 10 mph. And then, another.
With a heavier bike, you might climb a six mph for an hour and 40 minutes, zip down the far side at 60 mph and finish in ten minutes ... and now have to haul that heavy bike up that hill and it might take you two hours this time. And ... you get two minutes less recovery.
By the third hill, you might say "Eff it." You have already spent two-and-a-half hours climbing and have a huge hill left.
Meanwhile the guy on the lighter bike has climbed for three hours and is heading on that long final downhill. He will only hit 50 mph ... but he will be home soon. You will be out in the middle of the mountains, exhausted, trying to find cell phone reception so you can get a ride.
But yeah, your heavier bike might go a little faster on the downhills.