Thats not true at all. In plenty of wargames (or games with warfare) there is not necessarily a defender bonus, particularly if the defender is not fortified. Europa Universalis, Empires in Arms, Warhammer (if units are out in the open), Civ4 (no defensive advantage if not fortified) just to name a handful. I think Hearts of Iron and Victoria defender bonus comes only if fortified (or in cities). Been a long time since I've played Advanced Squad leader.
In many turn-based wargames, the defender doesn't even get a chance to do anything. Dungeon's and dragons style, Player A attacks a defender on A's turn, and then B counterattacks on B's turn.
Actually in most grand strategy games the defender does get a bonus.
Europa Universalis
In this game if I'm not mistaken, the attacker gets hit by a game mechanic called
attrition. This mechanic can whittle an attacking force down to a half or quarter strength. This means that you need a much larger attacking force to win in a war.
Empires in Arms
This is a game of where you pit the attacker's maneuver vs the defender's maneuver (outflank, cordon, echelon, etc.) Given that I'm not that familiar with this game I cannot say who has the real advantage. Maybe someone who knows can let us know based on two forces of equal strength, who if any has any advantage.
Warhammer
This may be one game that does not give any bonus to any terrain whatsoever (flat, hilly, etc.) There is no real "attacker" or "defender" as far as game mechanics are concerned. You can set up a scenario where you say "these lands belonged to the dwarfs and the skaven are attacking", but that is more for flavor.
I have never played the game but I have a boxed set of minis as well as the complete rules in my basement, i just don't feel the bug to go down and look it all up.
Civ4
Civ4 was never really considered a wargame. Plus CivV is now hex-based, 1upt (limited armies per area), and was supposed to be modeled off of panzer general. CivV is supposed to feel more war-gamey unlike Civ4.
Hearts of Iron
In this game the defender bonus is inherent in the units not terrain. There are three basic stats: 1) attack (soft, hard) 2) defense and 3) toughness; where defense > toughness.
In this game if you have 2 equal infantry, one unfortified in clear land and the other attacking then the defender will always have the advantage due using the more advantageous
defense number.
The attacker on the other hand will have to use the not so great
toughness number and will therefore take greater casualties.
And depending on the unit type (such as militia), some units can take up to 6x the casualties just by going on the offensive rather than just sitting there on the defensive.
Victoria
I don't know anything about this game maybe someone can enlighten us.
Advanced Squad leader
This game gives massive bonus to the defender. Especially in open terrain (in open terrain there is usually someplace to take "cover") the defender typically mows down attackers trying to get into position.
There have been numerous session with my buddies were I can remember my units with their SMGs/HMGs get LOS on the attacking forces maneuvering to gain a favorable position only to get KIAed (killed in action). The body count for the attacker can be massive.
If you read through the historical references in the rule manual that Avalon Hill put in most scenarios, you will find that just to get on even odds the attacker had to move in with 3:1 or 4:1 unit strength just to be on an even playing field vs the defender.
In conclusion
Most games give the defender a big bonus because that's what happened in history.
-The American civil war - the Union Army (the attackers) lost 2:1 in troops, despite having an army 3x the size of the South.
-Stalingrad - Massive soviet losses against the German 6th army under Paulus. The German 6th army was out-gunned, out-numbered, under strengthed, encircled, out of supply, starving, and no-where to retreat yet caused losses in the range of 2:1.