Why do I need a lawyer to draft my separation agreement for me?
Not only you require a lawyer but your spouse also will need a lawyer different from your own. If you do not each have a lawyer, then there is no way to ensure that the agreement is fair, that the agreement is in compliance with the law or that one party did not force or pressure the other party into signing the agreement.
It is of course physically possible for you to draft your own separation agreement, but without lawyers to represent each party and explain to them their rights, risks and obligations, a judge will not be able to ensure the circumstances under which the agreement was signed were appropriate and therefore will probably not enforce the agreement if it was challenged in the courts.
Parties are able to reduce costs by drafting their own agreement, having a lawyer look it over and edit it and then having the other party review the agreement with a different lawyer.