Well, it needs to be that high, because night.
When the sun sets it gets dark and the solar power is no more, and the only electricity they have is from batteries.
So it will consume the batteries until such time as the sun comes up. If they exhaust the batteries before that time they can avoid an early splash down by being high up to begin with.
Still, wonder how long this thing can glide at night? Higher altitude means less lift, no?
I tried some searches and it was surprisingly hard to find the information I wanted. Glide slope is the issue when there is no power and you are gradually losing altitude. So you have to aim the plane downward to prevent stalling and lose altitude at a calculated rate. This plane is surely mostly glider and has a very very shallow glide slope all of which has been carefully calculated given an expected altitude as solar energy goes away. I'm not a pilot and I'm sure some Freeper pilot will correct if needed.