Why don't model designers/manufacturers...?

I think a big part of the problem is that the joint between halves has to come exactly at the midpoint of the fuselage - whether vertical or horizontal.  Otherwise it won't be possible to get the part out of the mold.  Sometimes there's a panel line on the real thing at that point; sometimes there isn't.  But the manufacturer has to either split the fuselage at the midpoint or make it out of more than two pieces - and there usually are strong economic and practical arguments against doing that. 

Some shapes do lend themselves to being split top/bottom rather than right/left; a lot of F-14, F-15, and F-16 kits, are done that way, for instance.  (In fact I can't think of one that isn't - though sometimes the nose section is molded separately and split left/right.)

Once in a while modelers get a bit over-enthusiastic about eradicating joints.  If I remember correctly, the P-51 Mustang actually does have a panel line running down the middle of the top of its nose, for instance.

I understand that the Tamiya 1/32 F-4J and F-4E Phantom kits use a "sliding mold" technology resulting in a one-piece fuselage.  Intakes and fuselage bottom are separate.

BTW, the Me-109 was built with lap joints along the top of the rear fuselage behind the canopy, resulting in panel lines.  Modelers unfamiliar with the Me-109 would fill in the fore-aft seam along the top.  Ironically, modelers familiar with the 109 would have to include references showing the panel line whenever entering their buildup in contests with less experienced judges.

Dave

I understand your frustration Fred.  That is about my least favorite task too.  Like Dave pointed out, the 109 has a line the length of the fuselage spine.  Problem is I don't think the glue seam looks good enough to pass for a panel line.   I end up filling, sanding then rescribing the line.  That would probably be the case wherever that seam was.