One atom has very sharp absorption lines; it interacts strongly with light when light frequency is in one of many very thin ranges around absorption frequencies $f_{mn} = \frac{E_m-E_n}{h}$, but very weakly when light frequency is different.
When two atoms of the same kind are far from each other, they behave as above. But if they get close enough to each other, their interaction causes splitting of their energy levels $E_n$ into many more levels; one energy level $E_n$ of one atom can split into many levels of slightly different energies if the combined system $E'_{n,i}$.
When the atoms gets close enough, they often form bound states like molecules or crystal lattice. This makes the splitting effect immense and where there were no absorption lines, now there are many. This leads to coalescing of the absorption lines into absorption peaks and broad absorption spectrum bands with much less "holes" in it.
In a block of coal the atoms are very close to each other and are interacting in the described way. So the absorption spectrum consists of immense number of absorption lines of the whole interacting system, so experimentally it is measured to be made of continuous absorption bands or even broad continuous spectrum.
