Because there is a significant power gap. Any games ported to Switch need to be downgraded to function on the undocked unit, which is maybe a tenth of the power of the base model PS4 & XBO. That is not trivial. Technically demanding games are highly unlikely to ever be ported to the Switch due to having to downgrade the game to the system. Also, Nintendo's online will probably continue to remain a step behind XBL and PSN. We'll never see RDR2, or FFVII Remake, or Cyberpunk 2077, or the inevitable GTA6, or most AAA multiplayer-focused titles.
While cross-gen games do show that, yes, games can and have been ported between two systems of vastly different power levels, such games are released at a time when developers are just starting to learn the ins and outs of a new system. Cross-gen games are rarely so demanding as to where the new-gen version can take full advantage of the new hardware, and are typically outclassed by later games made exclusively and optimized for current-gen systems. Meanwhile, major AAA games being released now for PS4 & XBO are fully optimized for those systems and their superior hardware capabilities, their developers having long since familiarized themselves with those systems and how to make the most of them. In other words, doing something like putting CoD: Ghosts on both PS3/360 and PS4/XBO is not equivalent to porting Red Dead Redemption 2 from the PS4 & XBO to the Switch.
And this is going to get worse long before the Switch is ready to be replaced. The PS5 and Xbox 4 could be released as early as Nov. 2020. By that point, the Switch will be just over 3-½ years old. Games being made for systems that may have upwards of 10 TFLOPs of GPU power, even if they are being made to be translatable to PS4 & XBO, aren't going to stand a chance in hell of being ported to a system with maybe 2% the graphics processing power.
The Switch will probably see plenty of mid-tier titles, games that don't demand a lot of graphical horsepower. Smaller-budget games (e.g., Mega Man 11, BlazBlue Cross Tag Battle), indie titles, compilations of older games (e.g., Street Fighter 30th Anniversary Collection), and certain remasters of past-gen games (e.g., Skyrim, Dark Souls). And sometimes we might get the odd game like Doom or Wolfenstein ported to it. But for the most part, I expect very few big-budget AAA titles to find their way to the Switch. Just because it might be popular doesn't mean much. The Wii was super popular, but third parties largely ignored it except as a dumping ground for shovelware, and that's because of the power gap between it and the PS3 & 360. While they may be treating the Switch more seriously, third parties are still going to be putting their biggest games on the most powerful hardware, and won't be giving the Switch the same kind of games the PS4 & XBO get.