Japan has a highly developed industry, including a very successful car industry. Japan has a long history of building aircraft, they produce their own military jets, they build their own orbital rockets and satellites. Clearly they country is technologically capable.
And yet it appears that post-WW2 Japan has only build two indigenous airliners, NAMC YS-11 in the sixties, and then MRJ now (and MRJ is still being developed).
Japan did manufacture airliners and continues to do so. The Mitsubishi Space Jet is a narrowbody in production now with a backlog of about 150 to ANA, SkyWest and a bunch of other airlines. Mitsubishi also acquired Bombardier's CRJ narrowbody program, which has a seperate backlog of another 200 or so planes. Before that, there was the NAMC YS-11 in the 1960s, which had a run of about 200, some of which remain in use.
Building the planes themselves makes little commercial sense for Mitsubishi right now, mostly because it would be cannibalising itself. Mitsubishi is the leading supplier for aerostructures and exteriors, particularly high-tech components like wingtips. The Boeing 787 Dreamliner was a JV between Mitsubishi and Boeing, with about 35% work share from Mitsubishi. 40% of Airbus A380 production costs are paid to Mitsubishi. Consequently, the Space Jet has been specifically marketed as non-competitive with Boeing/Airbus.