It was announced via a memo released from gaming retail chain GameStop that Sony will be discontinuing sales of digital PlayStation 4 titles in retail locations. Pre-paid PlayStation Store cards; however, will still be able to be purchased at the retail level.
This is a cost-cutting measure for Sony as they will be moving all digital purchases of games to their PlayStation Store. This change will take place as of April 1, 2019. The final games that will still be available for purchase will be Days Gone and Mortal Kombat 11. Outside of those two titles, all other past and future digital titles can only be bought through the PlayStation Store. This also holds true for pre-orders of digital games. GameStop noted that they are working through a process as to what to do with those who have pre-ordered digital codes and will no longer be able to receive them due to this new ruling by Sony.
It should be noted that this only affects full versions of games. Add-on content is not affected by this and can still be purchased at the retail level.
Source: Gamasutra