Because they SO ARE. Look, if he's using that line to excuse a total lack of romantic effort, that's one thing. But if the guy we're talking about is a solid boyfriend, and shows you he cares in other ways, then I can't say he doesn't have a point.
Flowers literally GROW EVERYWHERE. For FREE. And to give them to you, he has to pay someone to kill them. It's really hard for a guy to understand what any of that has to do with showing you he loves you.
Let's say your man really wanted you to occasionally pay $14.50 for a bundle of lawn trimmings that he kept on his desk for a few days then threw away. Would you do it? Because I will hold you to that shit.
It's the thought that counts, not the object. If this guy's not delivering in the thought department, then you've got a problem. Otherwise chalk it up to an anti-establishment kick and make him pick some wildflowers for you next time you're cavorting in a field.