Because the law say so. Only an attorney can represent a corporation in court. It may seem unfair and provide attorneys with more work, but that has been the law for a very long time.
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"Why" is a policy question. Except for Small Claims court, which doesn't allow lawyers and requires principals to represent business entities, corporations and LLCs have to be represented by lawyers in court. It's one of the few areas of law that still treat individuals differently than corporations in terms of rights, since individuals are allowed to be "in pro per" without a lawyer.
Anpther way to look at it is that maybe lawyers influenced the rule to insure more of their own employment.
And maybe this sounds self-serving, but I'm sure judges appreciate this rule, since litigants in pro per rarely know what they're doing and cause delays and a waste of judicial resources.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
It is because only lawyers are authorized by law to act as a representative in court for another person. A corporation and other artificial entities act only through their agents, so anyone could claim to represent the corporation. If it turns out that the person files a lawsuit or defends a case for a corporation and it turns out to be false, if it is a non-lawyer then there is very little the court can do to the person, other than try to issue sanctions. They may even do it again.
But if the representative is limited to lawyers, who are officers of the court and regulated by the State Bar, if a lawyer files a lawsuit without proper authority to represent the corporation, the lawyer can be disciplined by the Court, the State Bar, or both. Also, this is the only way that the court can know that the corporation is being given proper legal advice. Otherwise, the corporation can say it was given bad advice by a non-lawyer and possibly the court will need to rehear past decisions.
It is not possible to establish authority for a particular corporate officer to represent the corporation - who would do it: CEO? VP of the division responsible for the alleged acts or omissions? The court would have no recourse against a corporate agent who failed to represent the corporation properly or without authorization. An attorney, however, is subject to bar penalties up to disbarment.