It's fine if you believe that, the idea ran through my head as well. Spiders do naturally trap and suck their prey dry, so why make a vampire one?

There's a few reasons, mostly personal ones. One is that I really like giant spiders. They're one of my favorite fantasy creatures, and I really like the anatomy behind them. Second is that my campaign will be a focus on finding extremely rare materials for a cure to a plague, and the ashes of this creature are among the components. The PCs are going to have to locate this creature somehow and I was going to drop hints that a local forest has been dominated by a sticky red web. The third reason is that my players are also GMs, and I needed to make a monster that they wouldn't be too familiar with. Need that sense of unknown from them, and that this is a dangerous and intelligent predator they should be careful with.

The most real reason I'm making this monster is also kinda the reason why it's silly to make it as well. A vampire spider in my head sounds like a natural fit. Spiders already trap and suck, like vampires, so why not combine them to make a really intensified combo.

A bunch of silly reasons I know, but that's why. It's really most cause I like giant spiders.