Because the "inland seashells" are from *multiple different time periods* -- and the times even vary from place to place. If there were a global flood, there would be a large layer of seashells in all places, all in a single strata that measured to the same time period. Furthermore, even single locations will often have a layer of sea shells, with a layer of geologic formation on top of that which can only be formed under *dry* conditions, followed by another layer of sea shells. Kind of hard to explain *that* by invoking a one-time global flood.
To even suggest that their presence is evidence of continental drift is preposterous.
And yet, that's exactly what the enormous amounts of geological evidence indicates, to the exclusion of any "global flood" hypothesis. Deal with it.
Here's what North America, Africa, and Europe looked like 390 million years ago:
(Image courtesy of the excellent Paleomap Project website, which is a good starting point for education about how the Earth has changed over time, and how we know.)
The light blue areas are shallow seas, which as you'll notice cover most of what are today "inland" areas.
Even as recently as 80 million years ago there were extensive "inland seas":
How do we know this is what the Earth was like 390 (or 80) million years ago? From careful reconstruction from literally millions of pieces of data acquired from such independent sources as paleomagnetism, linear magnetic anomalies, paleobiogeography, paleoclimatology, geologic and tectonic history, and the nature of various geologic strata from location to location.
Reconstructions of the configuration of the Earth in the past are based on such research and measurements as:
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Boucot. A.J., Xu, C., and Scotese, C.R., (in prep), Atlas of Lithological Indicators of Climate, Geol. Soc. Amer. Special Paper.
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