Guards are shit thats why.
They drop all of the weapons that they have like pinatas and they will bleed your MP dry.
People make guards for Vehicle Button to prevent vehicle dives, not for its combat ability.
This is one of those situations where on paper a unit may win but in actual gameplay it is a completely different story.
For example a Puma vs Su-85 in a 1v1 lane matchup like the one shown by OP the SU-85 would win every time but in the hands of a skilled player, using map LOS, flanking then the Puma can beat the SU-85 with ease.
For Combat I would rather use Airborne Guards or Shock Troops any day over regular Guards.
the thing about guards is that you WANT to drop a ptrs because you get a mosin and the loss of AT means really little for guards, if anything, cause its only effective against LVs and you lose against the likes of a p2, later on the ptrs become tickle machines at best. If you drop a DP though you gotta pick it up
Button is extremely situational, It IS more useful than the airborne guard's IL2 ability, but its purely defensive and is only effective when enemies drive tanks into melee range (the guards cant move and LOS obstruction auto break the button, so combat blitz just negates it). I have no clue why people even mention it
the guards have firing positions that basically give 25% free rate of fire, which is quite good, although it can be shut down a bit too easy with the abundance of grenades the enemy has to throw at you