If a company employs 20 people and 5 of them are programmers without which the company can't function, one leaves and recruits away the other 4 to go work at the same place that means the remaining 15 people are out of work.
Business owners have a lot of people's livelihoods at stake. It's about a lot more than just how much more another company is willing to offer you. That might not matter to you, but that's also why you get to see the policy before agreeing to work at the company.
If coming to work at my office and having the flexibility to recruit away all my staff is important to you...then you don't really have to sign the contract. If you do sign the contract then you're agreeing to the terms. It's not rocket science. Fair policy is that you get to decide whether or not to abide by it before I start paying you. You have the right to decide that policy isn't okay with you and not take the job. Nobody forced you to sign the contract and take the job. If it's a market problem, then I will eventually lose all of those employees because they will have plenty of other choices if they aren't happy working there anymore.
Or to spin this around into the language that you used, policies like the one I describe exist because a lot of people only have their own self-interest in mind and part of my job is to have the interests of all of my employees in mind.
