Exporting is intended for images for screen viewing, where ppi is irrelevant, and it is stripped out.
So an exported image has no ppi value. But some applications, like Photoshop, will assign a ppi value when you open an exported image. Photoshop has to assign a value to be able to display rulers and type correctly. It could be any number, but it happens to be 72.
To preserve the ppi number, use Save As instead of Export.
You can save in 300ppi but as Per Bernsten said, exporting has no ppi value attached to the image. The 72ppi or 300ppi or 1675 ppi is assigned by the application on opening the document if no ppi meta value is present. Photoshop assigns 300ppi others do different.