It may be pedantically observed that a raven is like a writing desk in many, many ways probably not intended to be addressed in the sort of extended identity logic applied to solving a riddle.

For example, clearly both are macroscopic physical objects. Both are manifested to us in more-or-less solid form, not liquid or gas, both are composed of of trillions of complex molecules and are decidedly lacking in material purity. Assuming the writing desk to be of wood (as most surely are), both are organic, and so the materials within both have known birth, life, and the inevitability of death. Of any particular raven and any particular writing desk, it may be posited with equal assurance of each that this particular one did not exist a few hundred years ago, and will no longer exist a few hundred years from now; the advance of human technology may ultimately extinguish both. Both have crunchy parts. Both will burn under the proper conditions, instead of melting or evaporating. Attempting sexual intercourse with either likely would yield unpleasant results, and if done in public, incarceration. In all the vastness of our Universe, both are found only in a narrow band of the outer crust of the third planet orbiting one particular small-to-medium-sized star in one particular small-to-medium-sized galaxy. (It may be that similar things do exist on other planets, but we would have to assume that some form of intelligent life has something comparable to our anatomy to be motivated to make something we would identify as a writing desk; and in the same vein, some very ravenlike creature may exist on another world, but at the DNA level, it would be no raven.)

Naturally, the original question itself relies upon pre-agreed conventions of meaning. For, the asker seeks not generic and universal truths of material objects; the unspoken question is, what characteristic is it that 'a writing desk' and 'a raven' have in common which sets them, as a duo, apart from other things? And, moreso, what words can be used in a manner that is especially descriptive of a characteristic of both, even if the word as applied to one has a completely different meaning for it's application to the other (both have 'legs'; the raven has a 'bill,' the writing table holds perhaps a pile of bills). But this leads us full circle to another generic point of familiarity, for both 'a raven' and 'a writing desk' are man-made symbolic representations, groups of letters strung together in the English language -- imagine how different the question would seem if it asked, 'Why is a cuervo like a bòrd-sgrìobhaidh,' though it would thusly be conveying the exact same ultimate meaning.

And so, here is my ultimate answer to the riddle: Why is a raven like a writing desk? Because the governing dynamics of our Universe are oriented toward producing planetary bodies covered with discrete macroscopic objects composed of collections of complex molecules; and because man chooses to define these concrete productions through ambiguous abstract symbols, that's why!!