............numbers their parts and arranges them the way they do?

I've like 50+/- italeri kits and I'm really frustrated about it. Part 53 is here but part 54 which connects to part 53 is on the other edge of the sprue or on another sheet!

Why cant they do it like tamiya does.

Not sure ... but at a guess, it may be to do with parts layout, being maximised, to be as econimically viable as possible. More parts fitted to a sprue might afford italeri to increase their profits and/or keep prices down. If this is a reason, and it helps keep Italeri kits the price they are ... I dont have a problem with it! I know italeri will take calculated short cuts and this could be another. Personally this is one Im happy to live with.

Frank is absolutly right.
Another reason for a certain layout of the parts is to ensure an optimal flow, for the liquid plastic in the mold, during the injection proces

could also be that some spruce's are used for more than one kit. I know Tamiya makes kits that by changeing out a spruce or two you can get a different variant or even vehicle..