The world has a finite number of Tyrannosaurs rexes. All that there will ever be have lived and long since died. And all of those that could have become fossilised have done so. What remains for palaeontologists is to find what of these are out there, but the number is constantly being reduced.
In order for us to end up with a fossil, quite a few things have to happen. Obviously the creature in question must become buried in some manner (in mud, in desert sands, in a tar pit) and in a form that will allow it to become fossilised (tiny insects can't leave much impression in coarse sand where the grains are bigger than they are). This must then be compressed and undergo the necessary geological processes to become a fossil. In then has to survive what may be hundreds of millions of years where it could be lost or destroyed from an earthquake perhaps, or simply buried under more recent rocks and inaccessible to researchers - we can only excavate fossils from places where they are exposed on the surface after all, and here there's the big problem.
They are exposed on the surface and visible to us because that rock is, by definition, eroding. If it was an area of deposition like a floodplain, more material would be being piled on top, or an area with little activity has little change (or perhaps has not much rock exposed, such as on a savannah), but where the rocks are on the surface and eroding, the bones and shells within will be exposed. Of course they will only be exposed for a limited time before they too are weathered away to nothing.
This means that every fossil going generally has a pretty limited timeframe in which it can be discovered. If a palaeontologist comes across it before it's on the surface, they'll walk right by, come by a little too late and the thing will have disintegrated and been destroyed. Only in that period in between (which depending on the fragility of the fossil and the rate of erosion could be just a few days) can a fossil be found and potentially collected.
Given how few palaeontologists there are out there, and the limited time we spend in the field (not to mention limited funds) it should come as no real surprise that while fossils are not only rare to begin with, we struggle to collect what is there before it is lost. Whole rafts of specimens, probably whole species and higher groups are being lost because we never found the few fossils that had survived to the modern era. Think of everything that has survived so long only to be hidden forever because it's under a car park, came to the surface in 1753, or was used as ballast, or for that matter, was collected by fossil dealers and whipped away into private collections.
There are, obviously, an awful lot of fossils out there. But they still represent a finite resource that is trickling away from us with each passing day. In my time in the field it's been far from uncommon to come across a mass of shattered and eroded bone splinter that just a few weeks ago would have been a magnificent skeleton if only someone had found it sooner. This is going on all the time all over the world and quite simply the only way we can really make a dent in the problem is to dig up more material, faster.
Protecting areas from illegal digging and development is an important step in protecting the fossil heritage of the Earth, but this only goes so far if the specimens in the area are not dug up and brought to the safety of museums and public collections. I have no idea what any of the figures are like, but I imagine that fossil collecting for academic research and museums has been increasing over the last couple of decades. Even so, it's easy to imagine that we are missing out on spectacular and important finds as we quite simply lack the personnel and resources to do much about it.
Fossils have to be incredibly lucky to make it to a point where they could be discovered and to miss out on anything by a matter of months over such colossal time periods seems almost comical. But it's a sad fact of life for palaeontologists that for all that fossils represent a tiny fraction of life that once lived, the specimens we do have are themselves a tiny fraction of the limited number of fossils that made it even this far.