Science

Evolution doesn't select for positive traits, just not-negative. If a trait doesn't strongly reduce the chance of reproducing, it can get passed down.

For example, humans have plenty of neutral traits (hair color, eye color), and even plenty of negative ones (Alzheimer's, arthritis, baldness, cancer, sickle-cell disease). But they're not so fatal that they don't get passed down.

Similarly, if neutral traits like cannabis including whatever chemical causes the munchies doesn't reduce that plant's ability to reproduce, it'll get passed down.

Evolution isn’t always necessarily immediately logical. Perhaps the cannabis plants with higher levels of cannabinoids developed idiosyncratically alongside their hemp cousins, and when humans discovered its psychedelic properties they began cultivating it for specifically that purpose. We know that wild-growing cannabis tends to have lower concentrations of cannabinoids than their cultivated counterparts - perhaps the concentration of cannabinoids is mostly due to us.

Cannabis flowers tend to hold the seeds for dispersal as well, and as long as they can survive an animal’s gut, any measure that makes the flower more desirable to eat increases propagation. Perhaps as cannabis developed higher levels of cannabinoids, humans and other animals noticed that they felt good when they ate and wanted to eat more flower, causing them to ingest more seeds and spread the plant farther.

It’s all speculation really, but there are some logical reasons why a plant with higher levels of cannabinoids would be either naturally or artificially selected for. It could also just be a random throw of the dice.

why would the cannabis plant want you to eat more?

I've never tried it, In some regions cannabis is being used as seasoning, and makes you eat more.

I assume it's just a coincidental interaction. I mean, it's not like yeast wants us to get drunk [citation needed].