As many have probably guessed, the modern entity called Atari has little real continuity with the original Atari thanks to a bunch of M&A nonsense. When Atari's last console (the Jaguar) flopped, there was little hope of recovery. Jack Tramiel sold Atari's assets to JTS, a failing hard drive manufacturer which didn't care at all about games, they just wanted the shell of an existing public company to reverse merge with in order to get listed on stock markets without going through an IPO. JTS laid off more or less everyone, and later sold the Atari name and IP rights on to Hasbro.
In turn, the reason it's a French company now is that Infogrames, a French game publisher, bought the brand from Hasbro to sell some of its games under, and later decided to rename itself to Atari.
I'd give good odds that Intellivision is much the same, except it's probably been a famous name being used by other organizations even longer than Atari.