Same thing that stopped you writing it yourself in Django at any other time in the last 20 years.

Most companies want to see outcomes quickly. They will always go for tried and tested solutions. Why would they risk building something on their own? There will be a threshold where companies are too small and the software is too expensive to buy, so they will go for something small that they build themselves. At scale, it will get harder and harder to maintain the solution themselves and they'd rather spend time focussing on their core business than to maintaining their in-house software.

People aren't going to be replacing Salesforce and SAP with this, they'll just use the Chat interface built into these products.

Don't forget too that a large part of what these products do is give auditing and compliance data...

You don't. But the management has been sold by the smooth fast talking salespeople. Then come the consultants to do the bespoke customization.