Because a global flood would produce as much, namely a record of death in sedimentary rock distributed throughout the globe. You do not know that the strata demonstrate chronological sequences, but assume all processes that produced the fossil record took place at the same rate, and in the same way, as those processes are taking place today. Not a bad assumption, but not necessarily a correct one either.
As will a gradual die off of species over time. Where the two ideas diverge is at the point of consideration of all the fossils found within a specific strata. Modern science examines not only the main fossils but the accompanying flora and fauna. This gives a good description of the local environment and the conditions that led to the deposition. Later and earlier strata are also considered when determining the history of the area. For a global flood scenario to be true the physics of stratification needs to be twisted and turned to explain the sequences found in consecutive strata.
You do not know that the strata demonstrate chronological sequences, but assume all processes that produced the fossil record took place at the same rate, and in the same way, as those processes are taking place today. Not a bad assumption, but not necessarily a correct one either.
There is a defined separation between the strata within a column. This is an observed fact (fact meaning data point). Material dropped from a height will accumulate on top on previously dropped material. This too is an observed fact. The upper material was deposited later than the lower material. Even during hydrological sorting this is true, the upper material settles out later than the lower material. There is a chronological sequence.