Ha @James Morris you basically said what I was starting to type out. Great minds and all.
I'd also toss in potential compliance issues. Audit and compliance issues often go hand-in-hand, but are not always the same. All the same arguments Jimbo makes also apply to any state or federal agencies that might audit you or you might have to submit reporting to. Maybe that doesn't apply to your company, but compliance audits are horrible if you don't have proper controls and reporting.
Also a weaker argument, but still a good one is that due to the reasons above, you're likely going to have to go to an ERP sometime. Homegrown systems don't tend to last as companies get to any size. The sooner you do the ERP implementation, the less of a mess you'll have to sift through to prep data and processes to move over.
And if you do that instead of a homegrown system right away, you won't have to go back and try to figure out what it's doing when you're moving to the ERP.
tbh a homegrown ERP really only makes sense if you're in a highly specialized LOB. Most of the major ERPs can handle most use cases. Will it work perfectly? Of course not. But it's way better to be able to rely on established companies and software packages when (not if) there are issues, rather than having to find the people who built your homegrown package. Good luck