Rishis are those people who through their deep meditation found meanings of these verses. The suktas on which a particular Rishi meditated is referred with that Rishi’s name. So Rishi are Mantra-Drashta and not Mantra-Rachayita (They are seers and not creators of Mantras)
Many mantras in Vedas have more than one rishis. Some have even thousand rishis. How can thousand rishis together make one verse?
The same Mantra in Vedas occurring at different sections have different Rishis. How can that be possible if Rishis were creators of mantras?
All ancient aarsh texts and even new Purans consider Vedas as revealed knowledge. Even Vedas claim so. Further there is no evidence whatsoever to make a claim that Vedas are written by Rishis.