#1 - Insularity of publishers. Bandai Namco is a Japanese company. Traditional PC gaming is nigh nonexistent in that market. As such, their view on the platform has been that it's not viable for non-service (MMOs, F2P titles, etc.) games. This has changed as the company's western branches have had greater say in what happens.
#2 - During Dragon Ball's early-mid 2000s western heyday, PC gaming was in decline in the US.
In that era, console and handheld gaming revenues were higher and still growing.
#3 - Difficulty with multi-platform development. Porting the PS2 games to PC during their initial run would probably have been a hard thing to do. PC gaming hardware was too fragmented and way too different from PS2's to make a port worthwhile when combined with #1 and #2. Xbox 360, PS4 and XB1 might as well be PCs, so ports from modern consoles aren't nearly as troublesome.
It should be said that Atari were planning a PC-exclusive FPS Dragon Ball game in 2002:
Most of the games in that article never saw a release.Finally, the PC will have a Dragon Ball Z game, too. Rinde isn't saying much about it except the game will be a first-person action game and that it is scheduled for release in time for the holidays at the end of the year. Rinde, though, hints that "we [Infogrames] will be utilizing a proven 3D action engine" to run the game. Quake? Lithtech? It's all conjecture, but Infogrames does publish PC games based on the Unreal engine.