I agree they won't do it, but it isn't laughable. I've worked at another FANG and have used a lot of the recruitment software you reference. That FANG rebuilt the software from the ground up internally and have successfully made their process much more effective to the point of it being a competitive advantage.
There's a tradeoff with building software internally and I agree is almost always isn't worth it. But the advantage is that the software has only 1 customer: yourself. That allows a level of product market fit that just isn't achievable with off the shelf software.
ERP Implementations go well for 2 types of companies. Type 1 companies have well documented, logical consistent processes and procedures that server as clear spec for necessary ERP modifications.
Type 2 companies admit that everything they do is wrong and they are willing to change all of their processes and procedures to match those of the ERP system.
Everyone else is deluded into thinking they can modify the ERP to adapt to their business. 10 months into the development cycle, team members realize that the firm requirement that A customer has One customer number is incorrect, and it happens to be a major customer, and there is not a workaround. Sales insists that it is a one to one relationship most of the time and refuses to believe this a big deal. Accounts Receivable thinks they could develop a manual workaround. Modification proceed until the money runs out, and the consultant move on to the next victim. 10 years later the new CEO want to embark on a modernization project for internal backend systems and the cycle repeats.
There's a tradeoff with building software internally and I agree is almost always isn't worth it. But the advantage is that the software has only 1 customer: yourself. That allows a level of product market fit that just isn't achievable with off the shelf software.