A target can be genuinely involved in a disease and still be a poor drug target, because druggability — also called tractability — is a separate question from biological validity. Druggability asks whether a small molecule can bind the protein with enough affinity and specificity to change its behavior, and the answer depends heavily on the protein's three-dimensional surface.
Proteins with a well-defined pocket — a cavity or groove where a small molecule can sit, make several contacts, and be held in a specific orientation — are generally tractable. Enzymes with a catalytic site and receptors with a ligand-binding site are the classic examples, and most approved small-molecule drugs act on targets of this kind. Proteins whose relevant surface is broad, flat, and featureless offer few anchor points; a small molecule has little to grip, and achieving both strong binding and selectivity against similar proteins becomes very difficult.
The distinction is structural, not biological. The protein may be central to the disease and still present no usable pocket. This is why the sequence-versus-structure distinction matters here: knowing a target's sequence tells you what the protein is, but only the folded structure — or a reliable predicted structure — tells you whether a drug-sized molecule has somewhere to bind. Some historically "undruggable" targets became tractable once new modalities or newly resolved structures revealed a pocket that had not been visible before, which is a reminder that tractability is a current assessment, not a permanent property.