"there are many people that are under contract with their respective employers that forbids them to contribute to any other project, whether it be open source or proprietary. Some contracts even say that anything you build while under contract is their "Intellectual Property.""
In many jurisdictions overly-broad restrictive covenants and even non-compete (post-separation) contractual terms are not enforceable, so one should investigate specific instances where contract terms are inhibiting one's action. An employer can put all sorts of terms in a contract that might not be legally enforceable (short of pissing off the boss--but that can happen without contract terms anyway). Check.
And this issue might be avoided up front by refusing to accept unreasonable restrictive terms and negotiating a contract that, absent extraordinary circumstances, respects the employee's autonomy and right to think and build outside the scope of employment.
From the employer's perspective, it would seem bad policy--as in off-putting to the most creative candidates--to force prospective employees into a contract where they cannot be creative outside of employment and where everything they make, without limit, belongs to the employer. That seems so unreasonable, even today, it should be repellent to everyone.