People are obsessed with returns, and less focused on final corpus. Say, you did an SIP, which generated 11% p.a. return in a year. Also assume, if you had made a lumpsum investment at the beginning of the year in the same asset, it would've generated 9% p.a. return.
Even if SIP return is higher here, corpus size would be higher for lumpsum investment.
It's easy to show this assuming initial investment of 120 INR vs 12 month SIP of 10 INR per month. In SIP, return of 11% would be applicable to each leg of SIP, but for varying duration.
Investing 120 INR all at once would've taken corpus to
120 * (1 + 9/100) INR = 130.8 INR over a year.
Investing via 10 INR / month SIP, would've generated 127 INR (refer to any online SIP calculator).
Despite being able to get higher return, the lumpsum one wins out, because the corpus experienced those returns longer.
It's possible to get higher return from SIP, and yet lose out in final corpus, compared to a lumpsum with lower return over same period.