PS4/XB1 games that have been in development for 3+ years now are obviously not going to be on Nintendo Switch day and date until there has been enough time for the development pipelines to get caught up. (namely, after such multiplatform titles were even greenlit for Nintendo Switch, which many might have not even been until early 2018)
Once the Nintendo Switch user base passes 40+ million, the switch cards come down in production costs, and teams have had time to catch up on projects, etc. there will be a much larger amount of games that come than what we are seeing now.
Now, all of that said, third party publishers have missed on some opportunities so far for sure. The most obvious stuff that could have been ported to the Switch in a much more timely manner has been quite sparse (not totally absent, but much much less than it could be).
Ports from 3DS and Vita should be much more numerous than they are by now. Yes, ports like Monster Hunter GU, Azure Strikers, Code of Princess, Retro City Rampage, Dragon Quest Builders, Ys VIII, etc. exist on the Nintendo Switch, but that is a very small amount of games considering how many more would be a good fit to up-port in a timely fashion (which would be profitable in the short term, and more importantly, help to establish a base to sell new entries to). Especially considering the Nintendo Switch has already outsold the Vita's lifetime sales (and will continue to do so several times over), is more capable in hardware and middleware engine support than both 3DS and Vita, and software sales keep climbing, etc.
And then, PS3/360 ports (again not totally absent, but very sparse compared to what it could be by now). Nintendo Switch, with larger performance and RAM overhead compared to PS3/360, do not need Panic Button’s level of porting expertise. That stuff can be outsourced to other developers for sure. I'm not talking asset remastered versions of these games, just porting the PS3/360 games with their current assets.
And yes, those ports take time, but after 19 months? There could certainly be more at least ANNOUNCED (with or without a trailer) by now.
So, so many sleeping at the wheel… I guess the silver lining is that a lot of indie studios are growing thanks to filling the gaps left open.
It will be interesting so see how and in what direction things change as the Nintendo Switch user base keeps growing.