Developed a fibrous tumor on the arch of my right foot. have had 2 surgeries to remove it but it keeps coming back. why is that?

Likely: These rumors have a tendency to come back. Likely the margin is not wide enough at time if surgery. Can consider radiation if no surgical option ?

Need clear margins: If the tumor is not completely excised, then a part of it will be left over that will grow back.

Plantar fibromatosis: Plantar fibromas are notorious for recurring. A radical plantar fasciectomy , where all of the plantar fascia is removed will lessen the chances of recurrence.