Why can’t we use the present continuous for the future with “be” itself by saying “I’m being there tomorrow”’?

Solution 1:

One doesn't ordinarily say

• *I'm being there tomorrow.

to indicate one's future location. True.
On the other hand, one doesn't ordinarily say

• *I'm being home today.

to indicate one's present location, either.

The problem is not with the future, nor with the rule about using progressive for future, though it's a stupid rule if it doesn't tell you that the progressive (or continuous) construction does not apply to stative predicates. And locatives are stative. Similar problems occur with other stative predicates:

• *He's being tall.
• *It's being warm today.
• *He's owning that house.

The progressive construction is for Active predicates -- verbs, mostly, like go, run, sit, rent, compile, write, ..., but also behavioral adjectives like dishonest, impolite, rude, ... You can say He's being rude but not *He's being tall. So, for active predicates, you can use the progressive to indicate future time, under the appropriate circumstances.

Executive summary: Don't take grammar rules too seriously. Most of the ones in books are BS.