Coverage is the real difference
A bed wraps light around you, so distance and angle stay roughly constant across your whole body. That consistency is hard to reproduce with a flat panel, where the parts of you nearest the centre receive noticeably more than the edges.
With a panel, whole-body exposure means repositioning: front, back, maybe sides, in sequence. That's workable, but it multiplies the session time and introduces variability every time you set up.