To be fair, the Umbrella bioweapons in the early games were just prototypes. "look at the cool shit we can do with this technology, imagine what it will do when we perfect it."
They were marketed mostly to people who don't care about them being uncontrollable. They're the perfect tool for Turkey to drop a couple into Kurd populated areas. Great for a dictator in Africa to cut apart the disloyal troops in a certain area. Chechnya giving Russia trouble? Just drop in some hunters.
But, critically, they can be sold to both sides. Are you ISIS and want to kill a bunch of Frenchmen? Why bother with building a dirty bomb when you can buy a single dose of "fill Paris with zombies" Dictator of your country got you down? Put some T virus in his kid's playpen and watch his own family eat him.
When we get to the later games, we see bio-weapons perfected. You can turn a population of civilians into instant, hard to kill guerilla solders who can still use guns and tanks and are ceaselessly loyal.