If you’re SAP, you build SAP once and you sell it. If you’re an author, you write the book once.
With consulting, you have to keep doing the same work every year just to stand still. You don’t own an actual product.
Many of these companies do build up internal know-how.
In case of big consulting companies, they have huge internal knowledge bases, decks, cases etc from previous engagements. In case of product companies with FDEs, the feedback from customers ideally trickles down to product improvements. In case of OpenAI, they can improve their models.
Whether that's actually valuable enough to turn a blind eye to the downsides of consulting, I don't know.