I think that it’s very simple: it IS weird that there’s a LEGAL CONTRACT among landlord and tenant and the landlord doesn’t declare this income.
Then you said that it’s not weird, that it happens all over the planet, that it’s super common because, you know, Airbnb.
Then I explain you how this business model is NEW and how governments are adapting to the new situation, and this means that NOW government do make Airbnb pay taxes, and Airbnb (and similar uber-like business) do apply taxes to each one of the transactions. Still governments (and companies) are figuring out how to adapt to the new situation.
So, in some way, Airbnb is collecting the taxes from the landlords and paying for them to the governments. However, in some countries landlords in Airbnb are being asked (or will be asked) to declare their activity in that platform.
So yeah, you can not say that it is normal that in Taiwan tenants sign off contracts with landlords and the later don’t declare the income, and to have another NEW, and ABNORMAL situation with business like Airbnb is no excuse, especially when IT IS BEING REGULATED NOW.
You are welcome 
Some random links about Airbnb and taxes and regulations: