Does anyone know what the driving force behind making large humanoids only have 5 ft reach?  It feels like a significant downgrade in their threat. 

Always free to give them a reach weapon if you want to.

I'm running a campaign and so i decided to put a green ogre in a swamp and the first things the players hear when they enter the swamp is "WHAT ARE YOU DOING IN MY SWAMP"

Monster Manual, Page 237

Ogres are able to carry around 570 pounds or 258 kilograms, all day, comfortably, they can lift, drag or pull around 1140 pounds or 517 kilograms and can throw your average medium humanoid as an improvised thrown weapon as per page 148 of the players handbook, so anyone hit by the character takes 1d4+4 bludgeoning damage, and the character being thrown takes that impact damage, plus a creature takes 1d6 bludgeoning damage for every 10 feet they fall and will be prone when they land. Ogres are strong enough to use the regular and long range for an improvised, thrown medium or smaller humanoid, so a normal range of 20 feet and a long range of 60 feet, making the attack roll to hit anything specific with disadvantage, and you can just divide the distance, rolling 1d6 bludgeoning damage for every 10 feet covered. 

Do you know what an interesting thought which came to my mind was? What if there was an enemy which was an intelligent Ogre? Think about it. This individual would somehow (leave it to anyone who cares enough to use this to figure out a reason) have intelligence which it could use to properly use tactics and trick people/adventurers. Especially if it pretended to be stupid so it could cause itself to be underestimated a bit. Just thought it was an interesting concept which I wanted to share.

There is an intelligent ogre, his name is Shrek

May i join this brilliant campaign?

Consider the Circlet of Intellect; an Ogre attuned to one has an Intelligence of 19. Wisdom & Charisma would still blow, but it would absolutely be up to creating some creative solutions and innovative tactics. 

So... just finished my first session of Horde of The Dragon Queen. Started part way where we were lv 4 and turned 5... and I beat an ogre... and got him to work for me. And just where we left off... there are two more ogres for the next encounter. 

when I was DMing my players had to fight 2 ogres and it scared them until I said at least they were not fighting Shrek and then everyone started singing All Star 

My first character, which I'm still playing, I made a human variant with ogre background. His great*4 grandfather looked for the least stupid and ugly one.

They can’t be too stupid, that picture shows the ogre wearing pants with a belt, using a leather pouch, o and wielding a club with metal spikes. That means ogres are either smart enough to undertake smithing, leatherwork and tailoring skills OR they’re smart enough to use money to pay someone to make them such things.

Not that the picture itself is indicative of much, but it strikes me that its just salvaged materials that are crudely slapped together made by other beings. The description does mention they do make loincloths and necklaces from whatever's left of their victims, and that really being about it. They can probably make their own weapons for the purposes of smashing things better, but its less sophisticated ironwork and more driving whatevers sharp and pokey through a big stick.

yes

would it be possible as a human fighter to train a baby ogre and ride on it when it reachs adulthood?

Quote from bragrman >>

They can’t be too stupid, that picture shows the ogre wearing pants with a belt, using a leather pouch, o and wielding a club with metal spikes. That means ogres are either smart enough to undertake smithing, leatherwork and tailoring skills OR they’re smart enough to use money to pay someone to make them such things.

 To be fair...the belt could be strapping from a saddle, and the pouch, a saddle bag. The loincloth could also just be a large piece of hide (your decision if tanned or not) that the saddle strapping is holding in place. I would actually spin it that the spikes in the club are just short swords or iron fence spikes that got stuck in the club from all the smashing it does.

There are plenty of ways to do all these things. Even some apes are able to use basic tools, so it's not too hard to imagine most ogres can reason it's easier to smash things with a club than with their hands. What's more, the fact that they often run with other races could be a source of their weapons, because more intelligent creatures can show them how weapons work/are more effective. They aren't gonna be working with special materials or other crafts, but they can swing heavy objects and listen to simple commands.

And then a fire breathing donkey-dragon baby hybrid flies out

Quote from Green_Legion >>

Do you know what an interesting thought which came to my mind was? What if there was an enemy which was an intelligent Ogre? Think about it. This individual would somehow (leave it to anyone who cares enough to use this to figure out a reason) have intelligence which it could use to properly use tactics and trick people/adventurers. Especially if it pretended to be stupid so it could cause itself to be underestimated a bit. Just thought it was an interesting concept which I wanted to share.

 Earlier editions had Ogre Magi, which were at home in the Kara-Tur setting and were exceptionally intelligent & could use some magic. In 5e, they’re called Oni.