What's the value in having enemies have slight differences in appearance as they get close to death? Will players actually notice or care about this? In a 2D games, it probably makes a lot more economical sense to add some kind of procedural effect to show this, such as tinting them red, making them flash, or showing a bright red outline.
He's not making a data-driven business app, where a shadow here or a custom icon there are nice but ultimately unnecessary. He's making a piece of software in a field where the small details can often be the difference between obscurity and massive success.
Of course, there are exceptions to the rule (Dungeon Raid, Doodle God, etc), but they became successful in spite of their lack of polish by having excellent and novel gameplay. When you're "borrowing" someone else's successful formula, you're competing with all of the crappy clones out there and need to put in a little extra effort to stand out.
Plus, he's making the game for fun, and it's a hell of a lot more fun to make something you're really proud of.
Counting frames is often not productive though, cut-out animations (which PvZ appears to be using) allow artists to render many frames from one base illustration and some extra work. They're not necessarily easy but they're often a large time savings over frame by frame.
I started my indie game 4 months ago, and i'm still adding a few touches here and there. Guess it's a creator's bane.
1. http://ifarchive.jmac.org/ 2. http://eblong.com/zarf/glulx/