They're less unified than the UK. Also less unified than just England, which is where HS2 is entirely located, much to the annoyance of Wales who still has to pay for it.

> their population is 6X smaller and GDP per capita is twice as high

2x per capita * 1/6 the population = 1/3 the available money

Gotthard is longer than all the tunnels in HS2 combined. Total cost: CHF 9.560 billion as of December 2015 (it opened 1 June 2016), about £6.5bn at the exchange rate at the time.

As of 2020, the budget envelope set out by the DfT is £98 billion. HS2 is not fifteen times longer than GBT, it is (currently) four.