Short answer: Because Jain Dharma teaches humility in tapas. When someone asks “sātamā?” (about the stage or count of a fast), the tapasvī replies “Dev‑Guru pasay/prasād” (by the refuge/grace of the Deva and Guru) to avoid ego and publicity, keep speech restrained, and preserve the pure intention of tapas.

A little more detail:

• Purpose: Tapas is for inner purification and nirjarā (shedding karma). Publicly stating counts or achievements can create mān (pride), yaśa‑lābh (seeking praise), and raag—reducing the spiritual fruit of the austerity.
• Vinay and gupti: Jain conduct encourages humility (vinay) and control of speech (vachan‑gupti/maun). Hence, an indirect, modest reply is preferred over “I’m on day X.”
• Common phrasing: In Shwetambar communities you often hear “Dev‑Guru prasād(he).” In Digambar communities you may hear “Dev‑Guru kṛpā se.” The wording varies, but the intent—humility and non‑display—is the same in both traditions.

If you wish to encourage a tapasvī, say simple blessings like “Sādhu! Tapasya safal rahe” rather than asking for numbers.

For a simple overview of Jain fasting practices and intent of tap, see: