Why, in the Solar System, all the mass seems to be concentrated at the centre? Is this a consequence of planet formation in accretion disks ?
It's because of the Sun.

It might be good if I give a quick overview on star formation before I get to the meat of the issue. Here's star formation in a few simple steps:


Giant Molecular Cloud forms. A large region of gas and dust, essentially a dense version of the interstellar medium, coalesces into an interstellar cloud. GMCs can be tens or hundreds of light-years across, enough to give birth to many stars. Within the GMC, some regions will be slightly denser than others.
A portion of the cloud collapses. A certain region of the GMC collapses, generally due to an outside disturbance. The most commonly cited cause is a supernova shockwave that compresses portions of the GMC, although close passes between galaxies have been known to incite star formation. My favorite example is the Cartwheel galaxy.
The region heats up. There is quite a lot of matter pressing in on what has now become a protostar, and so it heats up. Eventually, conditions become such that hydrogen fusion is possible. The protostar, now a pre-main-sequence star, begins to shine.
A protoplanetary disk forms. At this point, the star dominates this region of the GMC. Matter nearby is pulled towards it by the force of gravity, and a circumstellar disk forms. It may be composed of gas and dust. Eventually, small grains of dust collide and form bigger grains. Planetesimals form, then protoplanets, and finally planets.


The reason that there isn't more matter in a given stellar system is that the star dominates the surrounding area. It pulls in nearly everything around it during its early life. Much of the region of the collapsing cloud is made of molecular $H_2$, and so it is pulled in and used for fusion.

Now the question translates to 'Why isn't the protoplanetary disk more massive'? The answer is that when the disk formed, much of the matter that was in its inner reaches spiraled into the Sun. This is partly due to the Poynting-Robertson effect, where photons from the Sun pull dust grains in. Over billions of years, the star can accumulate much of the matter than was originally close to it in the disk.