Why didn't I get the 3-mile weekend walking challenge badge even though I walked over 5 miles on June 6...

Did I actually have to enter a "walking" activity like cycling, running or rowing?

Yes, it had to be recorded as the Walk Activity. However, if you recorded it under another type, like for example Hike, all you need to do is changing the type of the activity to Walk in Garmin Connect, and you'll perhaps get the Badge even now. Altough it is not guaranteed, since the Challenge was closed already. It is worth of trying anyway.

If yes, that's ridiculous..

Why ridiculous? What other activity type would you expect at a Walk Challenge? Or did you just walk without starting any Activity at all? That cannot work, since without starting an outdoor Activity, no GPS is used, and the walked distance cannot be measured (steps alone are not a reliable mean to measure the distance).

I don't see how total steps in a day isn't exactly the same thing as a walking activity...less the GPS map.  But I digress...

I don't see how total steps in a day isn't exactly the same thing as a walking activity...less the GPS map

Steps tell nothing about the distance. You can trigger steps just by swinging your arm staying on the same spot. GPS will confirm the distance better. 

OK, I didn't want to say it but you did...some people must cheat to get steps it seems.  Oh well...

I can get a decent amount of steps playing Beat Saber. I think as much (or more) than walking the same amount of time. The Garmin activity detect feature have guessed I was swimming a couple of times before I switched it off.

So yeah, getting awarded a 3 mile outdoor walk badge for playing VR, in my house for under an hour wouldn’t necessarily be fair. 

But on the other hand, know that if I ever get my 50,000 step badge from playing Beat Saber I’ll have goddamn earned it!!

Now I have the Michael Jackson song in my head
... just beat it...