Well,,,
It's more accurate
It's easier to mount optics on
It has better ergonomics
It can't suffer from the wood warping due to varying levels of humidity
No one has said in 'every way' but even you admitted that it was better.
Is there some way that you believe the M14/M1A is superior to the AR10 (or for that matter to the T48/FAL)?
When you look at (essentially anything) vs. the M14, you have to separate several different things in your mind, in order to do a fair comparison.
Things like what is best suited for the desired military role, and what is best suited to you or I as a civilian shooter, for one.
Another is comparing a basic stock rifle you can buy "off the shelf" against one with all the aftermarket extra you can bolt on.
And then, there are also those design features which you think important, which may not be to someone else.
And, also, be careful to compare apples to apples, only, if, and where possible.
I've used the M14. And fired it full auto. I've owned an M1A for decades. I've done range time with a couple different makers versions of the AR 10. I've fired the FAL, and owned a civilian version.
I've fired the G3, and owned an HK 91.
lets look at some of the points already mentioned, AR10 vs. M14;
"its more accurate"
Ok, I'll agree, today's AR 10s will give you better groups than a rack grade M14. Maybe better than a Match grade? OK. So what? Outside of paper punching and match shooting, what does that accuracy "advantage" get you? The M14 is accurate enough for combat use. So is the AR. Both shoot as well as I do from field positions, if not better. I consider it a wash, for combat use.
"its easier to mount optics on"
Absolutely. Point for the AR, IF you use optics.
Right now, its uncommon to find an AR 10 that has sights. You have to add them, or use an optic. The M14 has good peep sights, standard equipment.
"It has better ergonomics"
This is a personal matter for many of us. What constitutes "better"???
Simply saying the AR has better ergonomics means nothing. What (in your opinion) makes it better? Please, be specific.
"It can't suffer from the wood warping due to varying levels of humidity"
True, there is no wood to warp. A properly finished wood stock doesn't warp, or only warps a small amount. While this is a big issue with a bolt action, or any rifle fitted in the old style (not free floated barrel), with the M14, the GI wood stock fit is "generous" in many areas, and there are many synthetic stocks that avoid the issue entirely.
A couple of people have mentioned "reduced ammo" due to weight, or "large heavy magazines". When comparing AR 10 to M14, this is completely irrelevant.
There are some features of the AR that might be considered double edged swords. Opinions vary.
The enclosed action design. Makes it tougher for stuff to get it, BUT also makes it tougher for what gets in, to get
out.
Manual bolt operation. AR charging handle only works one way (pulls back), and the leverage area is small. M14 op rod handle works both ways, and has a fairly large handle to exert force on, if needed. Is this important? How important? Opinions vary.
Magazine reliability. Early gen AR 10 mags were notoriously unreliable. Later ones seem much better. M14 mags have a long proven history of reliablility.
Much has been made of the uncontrollability of the M14 on full auto. This is entirely true for anyone not experienced shooting it on full auto. Experience moves the M14 from "uncontrollable" to "very difficult" to control. And the receiver is (essentially) overstressed during FA fire, which has led to issues (cracking, etc.)
This is not the fault of the rifle, directly. It is the fault of where the development was stopped. Some civilian experimenters have found that a fairly simple change to the gas system can reduce the cyclic rate down to something that is both controllable and does not overstress the receivers. The military never bothered to figure that out.
The M14 is a good combat
rifle. The intent to make it a master of all trades, failed. Its too light to be a good BAR class gun (with the original cyclic rate), and too big and heavy to be a good assault rifle. In original GI trim, its length is a drawback when getting in and out of vehicles is important. (and so is the original (and shorter) M16 series., which is one reason we now use M4 carbines a lot).
Everything is a trade off, and what is vitally important to you might be "meh.." to me, and vice versa.
The M1A fits me well. For me, it balances well. AR, FAL, HK, all good guns, all with their strengths and weaknesses. I have had them all, and gotten rid of all except the M1A. Might be nostalgia, as much as anything, but for me, the M1A is the keeper.