Here's another myth of first-party software that few seem to realize: first-party software dominance among the best-selling games is the rule on consoles, not the exception. Nintendo is actually in the majority here, even though few seem to realize this. Let me give you some examples:
Genesis/Megadrive: Five of the top ten (actually 5 of the top 8) Genesis titles were published by Sega. The #1 and #2 games (Sonic 2 - 6.03m, Sonic - 4.34m) were both Sega first-party. No other game is even close in sales (next best is Mortal Kombat at 2.67m).
Saturn: The Saturn only had three million sellers, all published by Sega.
Dreamcast: We have six million sellers listed in the VGC database. Five of them were published by Sega.
Playstation: Five of the top ten (and again, actually 5 of the top 8) Playstation titles were Sony first-party: Gran Turismo (#1), Gran Turismo 2 (#3), Crash 2 (#5), Crash 3 (#6), and the original Crash Bandicoot (#8). Even on Sony's original platform, first-party titles dominate the top of the charts.
PS3: There are 14 million sellers on the PS3, and - surprise! - 7 of them are Sony first-party. Four of the top seven are also published by Sony (Motorstorm #2, Resistance #4, Gran Turismo Prologue #6, Uncharted #7).
XBox: Four of the top ten XBox games were published by Microsoft. But it's even more skewed that that: 4 of the top 6 games are from Microsoft. (Halo 2 #1, Halo #2, Fable #4, Project Gotham Racing #6). This is even more impressive given how few games Microsoft actually publishes.
360: Only three of the top ten 360 games are first-party, but 3 of the top 5 are from Microsoft. (Halo 3 #1, Gears of War #3, Forza Motorsport 2 #5). Again, this is more impressive because Microsoft very rarely produces first-party games at all.
What's the one great exception to the rule? The PS2. Only two of the top ten games were from Sony (Gran Turismo 3 and 4), and the platform was dominated by third parties. But it should be clear from the examples that this was highly unusual situation; first-party software usually makes up most of the top games on each platform. This makes perfect sense, since most people buy consoles for the exclusive games on them - which are usually the first-party games. Nintendo is more heavily first-party than other companies, but it's not nearly as dramatic as many seem to think.