WARNING: Potential SPOILERS follow!
Throughout Spider-Man: No Way Home, Sandman and The Lizard appear solely as CG characters; Flint Marko remains in his sandy form, while Dr. Curt Connors has fully embraced his reptilian side. However, during the final battle, both villains are returned to their human selves.
Did those scenes look at all familiar to you? Well, they should have, because Marvel Studios reused footage from 2007's Spider-Man 3 and 2012's The Amazing Spider-Man. When Flint finally reappears, it's through footage taken from that sewer fight with the black suit Spidey. However, instead of disintegrating after being blasted with water, it's reworked to make it look like his body is returning.
As for The Lizard, the sight of a clearly devastated Connors is lifted right from his defeat in The Amazing Spider-Man, albeit from what looks to be a different angle. It doesn't work quite as well as Sandman's human return, but what could the reason be for this old footage's inclusion? After all, both Thomas Haden Church and Rhys Ifans returned to lend their voices to these characters, so it's not as if they didn't want to reprise their respective roles for Spider-Man: No Way Home.
We'll probably get an answer at some point, but the fact that this movie was shot in the midst of the pandemic is probably the main reason (scheduling issues may also have played a part). With everything going on in the world, bringing the actors to set for one scene simply may not have been practical. It's also feasible they signed on late and that neither character had a voice in an earlier version of the movie.
You can check out some comparison photos over at The Direct, but let us know if you spotted this down in the comments section.