Strictly speaking, from a legal perspective, this isn't necessarily the case. There are many, many, many national 501(c)(3) organizations that allow local chapters to take donations (both in cash and in-kind) under the national organization's tax ID number. There's nothing inappropriate or illegal about doing so. However, BSA has apparently made a decision at a national level not to allow local Packs/Troops to operate under their tax ID number and to require significant donations to be funneled through the BSA nationally. I have my own opinions about why they do this, but I'll keep those to myself here.
There IS a perfectly legal and viable way around this little problem. A sI understand it, each Pack/Troop has a sponsoring organization. Sponsoring organizations themselves are usually 501(c)(3) organizations in their own right (churches, fraternal orders, etc.). If a large gift can't be accepted by the Pack/Troop itself, many times the gift can be made to the sponsoring organization, dedicated for use by the Pack/Troop.
The only downside to that loophole is that if, for some reason, a Troop/Pack changes sponsors, any donated property would probably legally belong to the sponsoring organization and not the Pack/Troop. My own experience is that such separations are pretty rare, but that might not be true everywhere.