Why do many Outlaw biker gangs have former military members?

It's the assless chaps man.... The assless chaps.

Because a fellow member of your chess club will not die for you if needed.

Major problem with the shape of the question. One of the so-called "biker gangs" arrested in the Waco fracas was a group of bike riders who restore classic bikes and had simply arrived at that location as the action began. There are all sorts of motorcycle clubs out there, the overwhelming majority are no more "gang members" or "outlaws" than VFW or Legion members. What you should be referring to is what is called "the one percenters". In actual motorcycle parlance, there is a vast difference between a "Club", an "Organization", an "Association", and the aforementioned "one-percenters". A "Club" exists because the members dedicate themselves to fellow members. An "Organization" such as the Patriot Guard Riders" serves a function, primarily but not exclusively on motorcycles. An "Association" is a loose group of people who get together just to ride together.
On top of that, there is a major sour smell coming from the entire Waco event. Far too many facts and stories are not tallying up. These COC (Confederation of Clubs and independents) meetings have been held annually for years without incident.
Is anyone besides me curious about the fact that with 9 dead and 18 or 19 wounded, there was not a single Officer injured in the least, and no "innocent bystanders" hurt? There is a great deal that is NOT being told here.

one of the original biker gangs the Hells Angels we're pilots from World War 2. after the war they started hanging out together and start up a gang called Hell's Angels using the same symbol was on the the bomber they were flying. and just evolve 20 days now. have a friend that is a vet and he also works for NASA and he belongs to the Hells Angels.

The underlying issue is that the attention the media is giving to the notion that many of these bikers are military vets, is merely the everpresent attempt to steer the "official" narrative toward demonizing veterans as enemies of the state, a threat posed by what the state deams liklely domestic terrorists.

That's a hard one. You have to know most vets are pretty much institutional guys. They like the sense of mission and belonging. I was just talking about this with another retired buddy (we both ride) and he believed it was mainly for the sense of camaraderie and order. Some want the additional elan of being in a "bad ass" outfit. Some want to revel in the memory of days gone by and wars long past done. Clubs don't have to be criminal organizations, but for those wishing to become outlaws there are clubs for that. There are combat vet clubs, "iron butt" endurance clubs - just tons of stuff. Find your happy space and join if you need to. There is also an awful lot of pleasure in riding alone.

As a VSO I see a lot of Vietnam vets. Many, in my opinion, are trying to regain their stolen youth or innocence. Perpetual youth.

post military need for some sort of brotherhood to reminisce perhaps

Many people join the military, because ironic as it seems, they are really rebels at heart. Either they wanted to do something 'different', they wanted the thrill of something out of the box, or they have serious beliefs and can't be told to not express them. Biker gangs, at least as what I've seen of them, allow these sort of individuals to express themselves in that fashion.

So they can make money! Nobody wants to hire a grumpy, PTSD having, bacon loving, flag waving asshole vet!