Short answer. Because Section 2 requires the complaint or information to be brought in the name of the People of the Philippines. A crime is treated as an offense against the State itself, not merely against the private complainant, so the State prosecutes it in its own name against every person who appears responsible.
What the law says
The complaint or information shall be in writing, in the name of the People of the Philippines and against all persons who appear to be responsible for the offense involved.
Rule 110, Section 2 — The complaint or information. Read the full provision →
A crime is an offense against the State
Section 2's requirement that the charging document be brought in the name of the People of the Philippines captures a basic idea in criminal procedure: a crime disturbs public order, not just the private interests of the person directly harmed. The victim may have filed the initial complaint, but the prosecution that follows belongs to the State. This State-versus-accused framing binds the prosecution to pursue the case regardless of the complainant's later wishes; even if the person who reported the crime later wants to drop the matter, the State's own interest in prosecuting an offense against public order is not simply erased by a private change of heart.
In writing, and against everyone who appears responsible
The section also requires the complaint or information to be in writing and to name all persons who appear to be responsible for the offense, not just the one the complainant blames. This means the charging document is not limited to the complainant's chosen target — it must reach everyone the evidence implicates. This is why preliminary investigation and inquest exist before formal charges are filed — to screen out those who do not appear responsible before the case is brought to court. This obligation binds the prosecutor conducting the investigation, not the private complainant, who has no independent power to include or exclude a suspect from the charging document once the prosecutor has made that determination.
What this means for a private complainant
If you reported a crime, you do not personally prosecute it — the case proceeds as 'People of the Philippines versus' the accused, with you typically appearing as the private complainant or offended party rather than the named party in the caption. Your role is to support the prosecution, not to control the litigation in your own name. This does not, however, leave a private complainant without any role at all; the offended party can still participate in the proceedings, present evidence supporting the prosecution's case, and pursue civil liability arising from the offense, even though the criminal action itself proceeds in the People's name, not the complainant's own.