Because the writer needs them to want Sandman. Otherwise, it's illogical and doesn't track, it makes no sense in the world of the characters.
And this set up also makes no sense, if indeed it was Ben.Also why should we care about Michelle when they obviously don't get along and the only set up for this date is Ben calling her?
How does Ben even know that Peter knows Michelle...
How did Ben get Michelle's phone number when she's a mafia consigliere...
How did Ben know that Michelle would fall for being negged in the first place...
And why is pranking Michelle - because if anyone is being pranked, it's her - supposed to inconvenience Peter other than the script says it does?
Also, Wells's streak of writing unlikable female characters continues. Not to mention, most people don't find negging to be hot.