Most likely, there isn't anything in Web Client that changed, thus there's no version change and so no option to Upgrade.
My thoughts are that it looks at the Version table in the GPCONFIGURATION database for Web Client, and as nothing changed there, it doesn't have any 'Upgrade' to do, compared to if you were coming from GP 2016 or a lesser version like that.
It is only Web Services for 18.2 that we were required to slipstream the MSP file in the Updates folder, for the install via the Configuration Wizard to go through, we've resolved this with Web Services for 18.3 and the MSP file is already there by default.
Looking at the WSInstallStatus table for Web Services, on a 18.3 environment, it does show a version of 18.3, so for Web Services there does appear to be an actual upgrade available.
Thanks