Urban planning and public transport should absolutely go hand-in-hand.
But on to your other point.
The key factor for transport use isn't just population or density. It's also the proportion of the population that uses public transport. And places that have more frequent public transport will have a higher proportion of the population using it than places with low quality public transport.
Imagine a city with just 100,000 people. But the local bus service is exceptional, and half the population uses it. That's a base of 50,000 people.
Imagine a city of 500,000 people. The public transport network there is average, so just 10% of the population uses it. That's 50,000 people.
Now imagine a metropolitan area of 5,000,000 people. The public transport network there is poor and infrequent. Only 1% of the population uses it. That's 50,000 people.
Three cities, same absolute number of public transport users, different modal share.
If you run frequent services, every 10 minutes or better, and services connect so that it's a two- or three-seat journey to everywhere in your city, you will have a much higher ridership than if it's an hourly bus service. That's with the same population and density.
Frequent bus services (once every 10 minutes or more) can also act as a feeder into a higher rail, light rail, tram, or metro services. In suburban, rural, and seni-rural areas, that extends the reach of your rail network.
Yes, higher density around railway stations is the best option. But where there is a lot of low-density suburban sprawl, frequent feeder buses are a good option.