Čechy is a plural-only noun.
I don't know, but Čechy being a plurale tantum is not that unusual. There are many toponyms that have plural form only; including older names for (Lower and Upper) Austria: (Dolní a Horní) Rakousy, Hungary: Uhry, Franconia: Franky and so on.
Ale pokud jsou Čechy plural, proč je použito is (jednotné číslo) Why is Čechy is and no Čechy are?
Because Bohemia is singular in English...?
It is, though. It's not an independent state, but it absolutely is a country. The two are not synonymous: for example, the United Kingdom is a single state, composed of three countries (England, Scotland, and Wales), part of one (northern Ireland), and perhaps numerous others as well (depending on how you classify Cornwall, which has a distinct identity and history from the rest of England, and the many many smaller islands).
It is only a historical country! The politic division of the Czech Republic is very different from the United Kingdom. Země Česká https://cs.wikipedia.org/wiki/Zem%C4%9B_%C4%8Cesk%C3%A1 was canceled in 1948. Since that time Czechoslovakia and the Czech republic was divided into much smaller districts (kraje and okresy).