The reason given by the Ubisoft employee is total baloney.

The reason they’re avoiding Splinter Cell as a full fledged game is it drives up interest and revenue in whatever other game they include Sam in. Both Ghost Recon Breakpoint and Rainbow Six Siege have included Sam Fisher, but have locked him essentially behind a pay wall. It creates headlines from big outlets like Polygon and IGN, usually along the line of ‘Sam Fisher showed up in Breakpoint. Here’s our thoughts.’

They absolutely could make a Splinter Cell game if the wanted, and they could do a good job of it. Both Dishonoreds showed that there most certainly is still a market for stealth games, and Fisher clearly drives revenue. They’re just choosing not to to bump up DLC sales of their other titles.