Very Few Spawnable Plants & Zombies

That's probably why. If you throw some math into it, you're getting much less consumables (and permanent unlocks) by purchasing more expensive packs. The benefit (and tradeoff) is that you're guaranteed a super rare and such.

• A Wonderous Pack of Greatness grants seven items total: five are permanent unlocks, two are consumables. Always. And these cost 35,000 coins.
• The Extraordinary Pack grands six items: four are permanent, two are consumables. But you're only getting a guaranteed rare. Cost hits at 15,000.
• Last, the Helpful Fun packs gives out five items: three permanents, two consumables. Only an uncommon guarentee in this one, but only costs 7,500.

I'll be honest, I started out buying Wonderous packs like I'm sure most others will. But as I spent my coins, I kept getting repeat uncommons. That said, I started buying only Helpful fun packs to (hopefully) eliminate all of the more common items so that the more expensive packs will yield better loot. It's a shot in the dark, but it's something worth testing in my opinion.

Back on topic, I can purchase five Helpful Fun packs for the cost of a single Wonderous pack. This gives me a total of 15 permanent unlocks, and 10 consumable cards. That's a ton of consumable items. I'm essentially decreasing my chances of getting the best items early on, in hopes that eliminating the bloat of the common cosmetics will help me finish my collection more quickly. Not to mention I have an unlimited supply of healing plants/bots, an underutilized tool that can really help you keep your vanquish streaks going!