There's an interview with the programmer for 2600 Space Invaders (Rick Maurer) on Archive.org.
There's some of it here at 17:18:
And here at 7:12:
He talks about sound, developing the graphics, and how these were effectively one-person games at the time. He sought out people to help with the graphics, and got no response, so he did the best he could. Also he mentions something particularly insightful that at the time these were only the second generation of games for the 2600 (4K), and while more advanced than the previous generation of 2K games, were still early and that future games would each bring new discoveries of how to make the 2600 do more things.
There wasn't a mandate from Atari that stated "This has to be as close to the arcade game as possible". They were under the gun to try to figure out how to program something that approximated the original arcade games on wildly different hardware (that was designed basically just to play Pong and Combat), get it done as fast as possible, and fit it into 4K. In its own way, 2600 Space Invaders graphics became as iconic as the arcade game. It became the killer app for the system.
At the time, I was disappointed that 2600 Space Invaders didn't look like the arcade game, but the gameplay made up for it. Pac-Man missed the mark in part because of the graphics, but the bigger problem was that it missed the "feel" of the arcade game. It felt clunky, imprecise and crude, and lacked the fun of the original. 2600 Space Invaders managed to retain the fun, despite looking different. But some early ports got much closer, notably Berzerk and Missile Command, and as time went on, and the programmers learned more about how to push the system (and Atari allowed for larger ROMs), the ports got better and better. They didn't have to spend as much time just figuring out how to display something, and could build upon previous work and focus more on refining and polishing the end result.