Sage has a computer kinetic brain, what means her mins works faster than a computer, can   instantly recall specific information with incredible speed and accuracy,  s he's able to store everything that she experiences in a photographic memory and retrieve it immediately, and   is able to track the probability of an event by piecing together stored data. She also is a Cyverpath. 
 
So theoretically she should be able to download and store all martial arts knowledge that exists-just like Neo in Matrix. She should be able to analyse an opponent and fight, while fighting, and come up with the perfect plan to beat anyone. In some ways like an upgrade version of Taskmaster and Lobo (the finding an opponents weaknesses part). 
 
Can s.o. explain to me why she isn't the human weapon I expect her to be?

Because reading it from a book or a harddrive is not the same as pulling it off with your own hands and feet.

^ What he said. 
Also the actual physical side of combat requires a decent level of physical durability. And while she's in "Peak physical condition". Her peak isn't the same as say Captain America's peak.

Well you sort of answer your own question with the theoretically part. We can either assume she has a limit to such abilities and actions (which makes sense, since their are certain problems one would encounter with downloading that much), or that the writer is downplaying her abilities, or their own writing for the sake of a perceive readership level. The comic fan. Why be accurate when you can be entertaining.  
 
Why isn't Rogue the greatest H2h fighter? She can absorb Sage, Cypher, Taskmaster, Domino, Longshot, Shang Chi, Elektra, Northstar and Colossus. Ouch lol 
 
I agree it would be nice if Sage's potential could be recognize more though too. Wall doesn't go well with her eyes. 

Marvel Wikia: " Sage is skilled in the martial arts, and is a formidable hand-to-hand combatant thanks to her having complete conscious control over her own body" 
 
So she already has the perfect foundation - so it wouldn't be so far to go. By the way, Taskmaster neither is peak human but he's up there with the greats (mostly by watching video footage). 
 
ps: her mind has no storage limits...at least thats what I read

Well sometimes your skill can't make up for brute force, see hulk.

I don't read that part about the limits at all lol, the foundation for your basis for example doesn't support that (if its the bolded parts) but thats cool, we all read into things different, I tend to think the comics will reflect mine. 

only thing i'm saying is the writers are inconsequent. you have a skilled h2h fighter, underline this fact with the  complete conscious control-part, and add a computer mind that beats every computer on the planet in speed and storage limits... 
 
to me that sounds like a perfected Taskmaster...but in the end the writers tread her differently. and don't forget the telepathic abilities (Mr X comes to mind)

Inconsistent? All writers that have ever been are, I agree with you, but like you say, Sage is not the only character like that. What about characters like Amazo, Rogue, Synch, Hope, etc etc, its just something we have to accept. (or at least something I accept)

Because she would be unstoppable and then different writers will have to nerf her down.Marvel doesn't know how to handle martial arts characters anymore and to be honest I don't think they ever did.They give characters stuff like photographic reflexes,knowledge of all martial arts styles ever created,mimicking abilities and ability absorption,mind reading,the massive use of chi techniques and then when it comes time for them to use the MA abilities they've acquired..their output is pathetic or nowhere near what it should be.How about not setting such ridiculous standards first? When Taskmaster was beating up on the Avengers he was just a great villain but as soon as he starts getting owned by people like Deadpool,Moon Knight,& Cage...now he has a problem with cowardice and then in Siege they completely acted like none of that happened.How can a guy who was damn near crying against Moon Knight because he was afraid fight Thor,and then Cap and Bucky at the same time with no issue?

There's one comic where her abilities to fight are likened to Wolverines at a skill level.

And I remember one issue where she just reads some guys martial moves due to processing speed and consequentially completely owns.

I'm too lazy to scan though. :P

Maybe it's because her body can't keep up with her mind.

Maybe it's because many other characters have more training, experience, skill and have won more fights.

Last time i saw her she had merged with the Panoptichron. Not many writers besides Claremont seems to want to use her.

@Chesapeake said:

It's time for her to come back. Where is she? Any X-book would be happy to have her..... come on!!!

Yeah, what Fesak said. At the end of New Exiles, she merged her body with their computer/system/thing.

I like this I was thinking the same thing. The thing about Sage is she is great in her own way. She can't be labeled as just a martial artist. Her greatest strength is finding the best possible way to defeat her opponent. Now that I think of it she is actually good they just need to show her h2h skill more. Like I said before she is more than a martial artist.

I miss Sage