[Get ready to read]
TL;DR at bottom

Because it doesn't fit the extended fiction from Halo's inception.

Bungie (or to be more correct, Eric Nylund) painted the Spartan II's as more stoic and mature, even to their young age.

Mission focused.
No time for chit-chat.
Militaristic hierarchy, following the chain of command to the letter.
Confident, not cocky.
Self assured through demonstration.

They were also subtle.
In the media prior to 343's stewardship, all Spartans, while wearing armor, looked EXACTLY the same, with the exception of minor insignia or emblem details.

Having read the extended fiction, the reader knew that while John, Fred, Linda etc all wore the same armor, there were subtle ways to tell them apart.

Having them appear as monolithic robots to the rest of the UNSC and Covenant helped further the idea that the Spartan's were special without even having to say it.

The more "flowery" fans also point out that this is what makes them broken characters, and to some extent, they are correct- being indoctrinated by the military from childhood to be the uber-soldier that humanity needs is certainly going to make them different, even alien to the average Human.

That, was the point.

The Spartan II's, for better or worse, we're able to say a lot by essentially not saying much of anything at all. It was through their actions that they were defined, and THAT, is why for fans such as myself, made them special.

Does that make sense?

Where it starts to change

The Spartan III's are not even dude-bros. They are a product of revenge. The candidates willing volunteers for essentially suicide missions. Disposable. Cost effective. They don't have a life time of forced indoctrination, they joined willingly at a young age (not as young as the S2's) after the Covenant either destroyed their planets or their families or both.

This gen has more emotion motivating their actions, and makes them more complex as characters in some ways to the S2's.

But, even in their portrayal in the extended fiction, and even Reach (the game, ret cons and all) the S3's were never like what we see with the S4's. I never once got vibes from Noble team that I was watching a bunch of over grown high school athletes. Jun and Emile, while more "animated" in some respects can have that attributed to the nature of their program and how they came to be Spartans.

In other words, even the S3's felt like all business, all the time, just like the S2's.

Where it starts to get worse

The reason the S4's draw flack, is that they are stereotypical to every other military group we get in a game. They are the gruff\cocky\smart ass types that we already have in CoD, Battlefield, Gears of War, Aliens, Predator, etc.

What 343 is attempting to do with the S4's has already been done and been done better by other media and other franchises.

On the flip side, it makes SENSE why the S4's are high-fiving macho-men because the overwhelming majority of them are former ODSTs, as S4's are staffed 100% by willing volunteers who are already young adults opposed to the indoctrinated kids that make of the S2's.

The ODST's were even painted that way by the Nylund and other authors, so 343 does have a legitimate reason to depict them the way they do, it's just that we as fans here, don't like that and would much rather see the stoic and reserved Spartan attitude.

The only in-game portrayals of this come from the Spartans in Halo Wars (which NAILED it for me, in terms of how other Spartans should look and interact with each other + other members of the UNSC) and Jorge (a 2) and how he worked with the rest of Noble team (all 3's).

: On a related note, I'd be utterly surprised if people said the same things if
: the Spartans were switched to female characters.

That would be an even more regrettable move. You should never shoe horn in a character based on race or gender just to make specific groups happy.

This is why Palmer sucks.

She is essentially a dude bro without a penis and written as such.

Linda, Kelly, and Alice are a few examples of how a female Spartan can be portrayed as a strong, complex character. Even Karen Traviss did an acceptable job writing about Naomi, which is saying a lot.
Osman is also another example of how to write an S2, albeit a washed out one.

TL;DR

The reason the MP intro\outro pisses people like me off, is these days, I see less and less of what look like SPARTANS and more and more of what look like modern day Pro-football (or Hand-egg, for those outside the U.S.) players who are wearing Halo armor and given guns and meet up for a game of ricochet.

Even the armor for that gametype looked like modern football gear.

It doesn't take a genius to see that this is the direction Halo MP is going, to be an e-sport, to cater to that mentality.

And since 343 has opted to make MP integrate into the story\campaign (which it never needed to fucking do), that means they have to write in characters like that to the Narrative, like Palmer and the rest of Team-whatever their names were in Spartan Ops.

The fact that I can't even remember their team name, shows how forgettable they are, and I consider myself a rabid Halo fan.

So to summarize, if shovin-buddy spartans is the direction the fiction is going, this is where I get off the Halo train.

My Head Canon will stop at Halo 4, with the Chief mourning the loss of Cortana and the new gen taking over.