If they are in their hides, any heat source from above is going to be blocked by the hide, or at least, the warmest part of the enclosure will be outside of the hide.

Not having belly heat underneath the hide requires the snake to choose between the highest heat and security.

Radiant heat is also harder to regulate due to thermodynamics. The sun usually gives a higher surface temperature than air temperature. Same with any radiant heat source. Thats why I use my radiant heat sources to maintain ambient (78-80°) rather than as hot side heat, where trying to make the inside of the hide 88-92 could result in surface temperatures of 95-100+ in the enclosure. Especially if you have taller decorations in your enclosure.