A few possible reasons...
1) The lease will be extended, so there's more value
2) There's more to the deal than just the lease. Assistance for the home-grown SSN project could be paid for by such deals.
3) It's not like we have a choice. Other than Russia, who else has an SSN they're willing to lease out?
1) What else is Russia going to do with the former Nerpa? It is in need of a refit which they can't afford to do buying a new class of attack subs.
2) India wants an SSN that they can test and prove indigenous subsystems, but it is better to just build your own which India has more than proven they can for less money and full ownership.
3) Of course you have a choice, build your own. The subsystems of the Scorpene are more advanced than Soviet era subsystems if you need experience with the best.