"Leave If You Do Not Like It" Is One of the Rudest Things To Say
Whether it be about a country, state/province, area, region, city, town, etc. this has to be be one of the most dismissive, rude, aggressive, and frankly ignorant things to say.
Many people might actually not have an issue or even like the place as a whole, and just have issues with a few things. Also, it doesn't even consider that moving can take time, cash, fields might not be available in some places, etc.
It also shows you really have nothing more to say since you, likely, are not well versed in the topic.
In some cases I can get it, such as if someone is being outright hateful of everything about a place: If someone has the means to leave and people just say "well hey since you can why not save up and look elsewhere?" that also is fair enough i'd say. Mostly, however, it's just a way for a person to dismiss someone.