We must not forget that Sega also made games for the Atari 2600. Only after the death of the Atari did Sega go on to produce its own console systems. (Some Sega Arcade games were also released for the Atari but Activision did the majority of those).
Sega Atari 2600 Games
Many of the above titles were also released for the Atari 5200 and the ColecoVision.