LM5085

LM5085 should work in this case. Webench must have an error.

Same for Vout=7.5V and Iout=8A?

webench has lots of limitations that reject designs for various corner cases.  I'll move this to the webench forum.

Hi Eli,
WEBENCH® could not create an LM5085 design because it could not recommend a FET based on the calculations with the design inputs.  The calculated temperature of the FET exceeds the max temperature (which includes a safety margin buffer).
It may be still possible to create this design, but WEBENCH is not recommending a solution at these input and output conditions because it did not pass this temperature check for the FETs.