"Why are Chinese OEMs so against global launches" - because it's difficult for Chinese companies to sell their stuff in the US. As for the people living in other countries, thet can just get what they like on Aliexpress.
OPPO's upcoming premium phone, the Find X8 Ultra, was recently rumored to launch next month. The company has now refuted those claims, confirming an April debut instead, but it appears the flagship will not be released on the global market regardless.