Because Oracle cannot install to /usr/local/oracle? I don't see the value of /opt.
Especially some commercial and proprietary software that runs on Windows, HP-UX, SCO, AIX, Solaris and Linux tends to go the "easy way" of packaging and just put stuff in one place on every system.
This is merely a convention, not set into the 2.3 standard apart from the naming 'tertiary hierarchy' otherwise implying this, but it's nonetheless a widespread enough convention — notably in use by every single configure/make/make install (and more) source distribution out there, whose default PREFIX is /usr/local — that we can expect it to be standard behavior.
I also prefer having packages with their own hierarchies under /opt, rather than /usr/local/<package> -- in general stuff under /usr/local should put their binaries in /usr/local/bin -- their manpages under /usr/local/share/man, headers and libraries in the corresponding places -- so that I don't have to mess with my PATH settings to be able to run a command, look up a man page or link against a library.
Sometimes software isn't packaged for use on a posix-like system -- and then I might have to do some dancing to get it to work -- I usually prefer having such programs under /opt/<program-version>.
I do have another folder under local: /usr/local/xstow -- so I can easily compile packages and manage different versions under /usr/local/xstow/package-x.y.z.
Most vendor supplied software is by tradition installed in /opt, for which we should be forever thankful because most software vendor wouldn't recognize a properly packaged piece of software even if it jumped up and bit them in their arse.
Oracle was in the early days installed in /u01 with data- and log-files spread out over separate disks with mountpoints normally named /u02,/u03 and so forth. It isn't uncommon that this naming convention still partially is used on oracle installations.
Today oracle published a standard called Optimal Flexible Architecture (OFA) where all oracle products should be installed under a common top-level directory.
The oracle universal installer isn't as horrible as it used to be but it isn't a well behaved rpm/dep installation either, at least it claims to have heard about LSB-directory structure even if it doesn't follow it very well.
Similarly for /srv
Remember: the filesystem hierarchy and your underlying storage don't have to correspond.
There are other tricks which can be accomplished by union mounts or similar foolishness.