They don't need to make their own malware, flood the market to sell the products. The approach is wrong. In order to understand this, you would need to go to
http://maec.mitre.org and understand a lot of what's going on. In a nutshell this is the issue:
Malware Signature
1 + 1 = 2
Attacker
one + 1 = 2
New Malware Signature
one + 1 = 2
Same attack + attacker
one plus one equals 2
New Malware Signature
one plus one equals 2
Same attack + attacker
b25lIHBsdXMgb25l
No matter how they want to attack the heuristics, its a guessing game based on what they KNOW. They can never see/know/understand an attacker so there is a lot of assumption based on known knowns. So attackers will ALWAYS have an upper hand. The keys isn't to rely on malware/AV companies, the key is to understanding your network, applications and patterns. E.g., any baseline traffic would yield anomalies in sites visited, bandwidth consumed and so forth. You start seeing things leave your network destined for say China at 3am... Its something you should be quick to look at. Same applies for ANY connection LEAVING your network when say, there is no one on a particular machine. HIPS also help here but running say Tripwire or Samhain in an enterprise can be a headache