Although very often the present perfect simple and progressive forms can be used interchangeably. Why is the simple form necessary here. "They have completely restored their home " is it because of "completely "?
Without a context, it is not possible to say that the present perfect is required, necessary, needed, or should be used. The sentence is also acceptable in other tenses : " They are completely restoring their home." "They will completely restore their home." "They completely restored their home."
The only tense that is dubious from a meaning aspect is "They completely restore their home." Restore seems to be an activity verb and to me in this case feels better with a progressive aspect. This isn't to say the present aspect cannot be used with it ("They completely restore homes.") but when you add "their" it makes the sentence specific in a way that doesn't seem to support the "fact" sense of the present aspect, which is why home is plural.
is it because of "completely "?
Nope. You can also say "They have partially restored their home."
A perfect tense doesn't indicate that the action completely attained the goal, just that it isn't going on any more.
I have eaten dinner. Right now, I'm not eating, but I'm in the state of having eaten: there is food in my belly and not in my place.
"I used to do drugs. I still do, but I used to, too."
— Mitch Hedberg