The reason you can't find a "pure firewall" much anymore is simple. Reviewers.
To me, pure firewall means state aware packet filtering - purely about traffic - i.e. it will look at source and destination IPs, protocol and ports use that to make a decision whether to let the traffic go through or not, in a chain or rules (that's what ghostwall is. That's what the old kerio and tiny used to be). Using a "pure firewall" on very restrictive rules should mean that no surprise traffic can get in or out. Of course a clever virus, or spyware, or a trojan, if it gets itself installed on the machine, can still hijack another software which is known to be usually trusted to get out. And that's normal, the firewall still did its job, watch the integrity of the network, another tool or two should watch the integrity of the system.
But reviewers and "obsessive" security power users started saying a firewall had failed if it didn't catch a browser hijacker, or a trojan... And the list of breaches a firewall is expected to catch keeps growing, year after year. They're doing the same to spyware detectors and virus scanners, too.
Whereas earlier people would have had a "pure" firewall, a virus scanner, maybe a trojan protection or intrusion detection and a spyware tool, now people tend to have a firewall that also does trojan and malware, a virus scanner that also does trojans and malware and more and more some virtualisation, a registry protection tool which also does a bit of firewall... All overlapping in features more and more. No wonder they conflict.
In a way I want several small tools that do their distinct job very well. I don't want 4 tools that are fighting to steal each other's job.
Edit: i did a quick search and you will find out that any review of firewalls nowadays centers on non-firewall features such as leak tests. That's what most firewall makers center their efforts on nowadays, making sure their system detect the leak tests, sometimes by cheats, but mostly by watching, scanning and analysing everything that happens between executables on the PC. This is quite slowing on the PC, in the end. And in a "one in all" product you can't turn off the anti-hijack tools on their own when you want, for example, to play a game.