Because there is no English sound corresponding to an unaspirated "p" (ป). Of course, it's possible to use "p" for the unaspirated ป and "ph" for the aspirated พ. But again, people will try to pronounce "ph" as "f", as heard many times (Fuket instead of Phuket lol).
There is simply no correct way to transliterate Thai into plain English letters without creating a specialized transliteration system where letters would have a new meaning, different from English.