Why MySQL doesn't use indexes in view with union select?
Because it is a kind of bug in the old MySQL version. UNION effect using indexes.
In the versions older than the 5.7.3 version of MySQL, UNION statement works with a temporary table. It means, first of all, your data move to the temporary table and read them from the temporary table before that implement filtering.

The server no longer uses a temporary table for UNION statements that
meet certain qualifications. Instead, it retains from temporary table
creation only the data structures necessary to perform result column
typecasting. The table is not fully instantiated and no rows are
written to or read from it; rows are sent directly to the client. The
result is reduced memory and disk requirements, and smaller delay
before the first row is sent to the client because the server need not
wait until the last query block is executed. EXPLAIN and optimizer
trace output will change: The UNION RESULT query block will not be
present because that block is the part that reads from the temporary
table.

Check out version 5.7.3 optimizer fixes.