Yeah Titanium is not easy to weld..

I grew up on Long Island near where Grumman built F-14 Tomcat fighter jets for the U.S. Navy in the 1970's. A friend of mine worked on the production line and he tells me the titanium assemblies for their particular application can only be welded using a computer-controlled electron-beam welder. Apparently that method of welding does make quite a strong joint-- So strong in fact that the titanium load-bearing wing boxes on the F-14's swing wings has never had a failure during the fighter jet's service life (the Tomcat's motto: "...Anytime, baby!" yeah!).

I guess bicycle manufacturers can't be bothered to invest in electron beam welder tooling for making titanium frames.