Because there's a separate clause in many trusts that addresses only income, like
After payment of all expenses of administering the Trust and maintaining the Trust Assets, any part or all of the net income of the Trust may, in the absolute discretion of the Trustees, be retained or paid to or applied for the benefit of the Beneficiary to be used in furtherance of the Purpose, at such intervals and in such amounts as the Trustees from time to time deem requisite or desirable and/or to comply with any applicable legal requirements. Any net income not paid or applied shall be added to and become a part of the principal of the Trust.
