Why land-based fighters are not all carrier-capable

Should all fighter and attack aircraft be carrier-capable to avoid duplicated development?

No. The structural reinforcement needed for repeated carrier landings would impose significant cost and performance penalties on aircraft that only need to operate from land bases.

A carrier landing is a controlled crash repeated throughout an aircraft's service life. The strengthening and engineering needed to survive that cycle add weight, cost, and design compromises that are unnecessary for many land-based aircraft. Making every fighter carrier-capable would therefore burden aircraft and budgets even when most examples never went to sea. Separate naval and air-force requirements can remain rational despite apparently similar combat missions.