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The first Sega System, the SG1000, was a straight up Colecovision clone, and they copied the Colecovision controller minus the number pad. So it was a joystick and two buttons via DB9 connector. The Atari DB9 connector in the basic interface they used has only seven lines, which they used as such:

• Button 1
• Button 2
• Right
• Left
• Down
• Up
• Ground

The controller just grounds one of the buttons to register a press.

See here for some more info.

https://nintendosegajapan.com/2015/12/29/adding-a-sega-controller-adapter-to-an-original-sg-1000-home-made-jc-100/

In the wake of losing to the revolutionary Famicom, they iterated on the console rapidly.

The SG1000 II moved to Famicom style control pads, but was still limited to the same controller interface.

Next Sega added an extra video mode to the same console design and released it as the Sega Mark III. This was later released in the west as ‘The Sega System’ and eventually branded ‘Sega Master System’ (which was originally sort of the name of a particular bundle, like the NES Action Set). But it still had the same Colecovision controller interface.

The next iteration was project Mark IV and it integrated some Mark III attachments: an FM audio module and 3D glasses module. It was released in Japan as the ‘Master System’ (but the motherboard is still branded as M4). Still the same Colecovision controller interface.

Their next console was developed as the Sega Mark V, which iterated again on the same basic design but added a new an X68K processor and an upgraded FM chip similar to their arcade games. It was eventually released as the Mega Drive (the motherboard is still branded ‘M5’). They managed to add two more buttons to the controller with a simple method - make the dpad have a rolling mechanism so you can’t hit up+down or left+right at the same time, and then those combinations were mapped to start and C, but otherwise it was still the same Colecovision controller interface.

Finally, to keep up with the SNES controller that has enough buttons to play Street Fighter II and Mortal Kombat properly, they added a multiplexer to allow three extra buttons for the six button controller. This was the first real upgrade from the Colecovision controller interface.

The Saturn was actually their first ever clean break from the Colecovision linage, and it just kept the 6 button Mega Drive design and added SNES shoulder buttons, but it finally dropped the ancient 'ground the line' design.

So basically no select button because it was all built on iterating their Colecovision clone, the SG1000.