find works by evaluating the expressions you give it until it can determine the truth value (true or false) of the entire expression. In your case, you’re essentially doing the following, since by default it ANDs the expressions together.
-name "*.xyz" OR ( -name "*.abc" AND -exec ... )
Quoth the man page:
GNU find searches
the directory tree rooted at each given file name by evaluating the
given expression from left to right, according to the rules of precedence (see section OPERATORS), until the outcome is known (the left
hand side is false for and operations, true for or), at which point
find moves on to the next file name.
That means that if the name matches *.xyz, it won’t even try to check the latter -name test or -exec, since it’s already true.
What you want to do is enforce precedence, which you can do with parentheses. Annoyingly, you also need to use backslashes to escape them on the shell:
find ./ \( -name "*.xyz" -o -name "*.abc" \) -exec cp {} /path/i/want/to/copy/to \;