Hi krass,

re:
> But I cannot understand (find the reason) why the surfaces do not merge into a solid. You can view?

Your object is already a solid, the file you posted contains one object which is a solid:



There are some messy areas in these juncture spots though:





It looks like there was some 3 sided networks done in those spots because the surfaces are awkwardly collapsing down to a point in those spots:



I'd recommend to not use Network in this manner to patch in 3 sided holes because it's difficult to get good shaping when you have collapsing poles like this.

Instead it would probably be better to build a blocky initial result and use fillets to round off transition areas rather than trying to construct surfaces that have a transition between shapes being baked into them.

- Michael