Almost all software could be rewritten to be more concise. But SAP's actually too big to have one architect get an overview of everything and then conceive of an optimal solution. Anything with its level of complexity will have to have evolved to get to that point.

And if you managed to get it down to only 10% of its current size, it'd still be several times larger than the Linux kernel.

SAP makes copious amounts of money, so naturally people have tried to replace it. So far, they have not succeeded.