Chemistry between actors is one of the most difficult things to fake in a movie. You can CGI in monsters and you can cleverly edit around an actor's abrupt weight gain, but there's no post-production tool that lets you magically create a spark between your two leads. It's either there or it's not.
You might think that actors - ya know, those people who are paid to pretend to be any number of things - might be able to utilize the skills of their craft to fabricate some believable chemistry. But more often than not, that simply doesn't pan out.