They've got a scale of how enforceable they are. Certain aspects are easily enforced, other aspects are difficult.
They can't block you from working in your chosen field, those get thrown out immediately in the courts. But depending on the details --- and those details are critically important --- they can preclude you from specific jobs or from specific activities. They are more heavily enforced when the person is of some prominence, or when they are highly associated with the brands or the company. They are more heavily enforced when the competition is highly visible, or when the competition rivalry is widely known (e.g. leaving Coke to work at Pepsi, leaving ABC to work at NBC), or when the competition is specifically named in the contract (although that can also backfire in enforcement when a company is using it in an abusive way). They need to have a limited duration, usually under 6 months maximum. They need to have a limited geographical scope that makes sense for the job. Usually they also require compensation for the time they're in effect; if they prevent you from working for 3 months they generally need to compensate you for those three months.
A leading news anchor on a city's major news show moving over to be the star news anchor on their direct competitor in the same city would be a problem, but moving to a news station in a distant city would probably pass legal scrutiny. Often when newscasters change companies there is a time when they are off the air, such as six months, to help with non-compete.
Within this industry the most common time they are enforced is when a group of people leave a company to start a new startup studio and they have a game that directly competes with one they were working on. Less commonly (but still occasionally) they are enforced when a studio lead is headhunted away to another company in the same geographic region.
Note that non-competes also tend to cover other facets like trade secrets, contact lists, and some specialized knowledge. When changing jobs be VERY careful before using any of that specialized knowledge. Check with HR and/or legal before saying "I've been working with this other group up until recently, I'll text them and get the new company hooked up!" The more time passes the better the situation is. Enforcement of agreements looks at how likely it is that you are abusing your past relationship, including contacts and goodwill established at the past job.
There is a second type of agreement similar to a non-compete. Non-solicitation agreements are a similar agreements generally between companies that they won't hire other workers, or between people if you're in a role working with business contacts. These also have a sliding scale. The most enforceable agreements protect trade secrets. People can still leave the job and change companies, but not for purposes of stealing specific key individuals, specific contacts, or specific knowledge. For example, contractors and subcontractor companies tend to have non-solicitation agreements with the hiring company, so someone working as a contractor for one company being offered a job by the company they were being contracted for can be a problem. Another example, getting a new job at a competing company and then actively hiring a former co-worker can be a problem or be fine, depending on the details. If they post on facebook to say "Hey everybody, I was just given a huge pile of cash to work here, twice as much as I was making before, come work here too!" that's fine because they aren't soliciting specific people or specific information or secrets. But "Hey Bob, you've been cooking for them for six years and know every recipe, come help us in our kitchen to formulate something 'new'.I'm sure the food will look very familiar ?", that one is going to have a hard time passing legal tests. These also have time limits, geographic limits, etc.
All that said, it costs time and money for a company to attempt to invoke a non-compete agreement or non-solicitation in the courts. Even if they are angry or upset about somebody leaving they need to determine how angry they are. If they're angry but not $50K+ angry, they won't enforce it. They may say or do things that make life a little more difficult down the road, and even little comments to former co-workers (like "he quit right when we needed him most") but not actually sue. But if an entire team was lured away, or if major clients suddenly vanished and followed former employers, the company won't hesitate to pay the legal fees.
