Most use it very intelligently. I’ve seen some amazing automations and I’ve seen countless simpler businesses running vanilla SAP as they should.
Basic business logic is built into SAP - but because it covers all aspects of business, it’s a massive tangle of rules.
For instance, let’s say you get an invoice from a vendor. Now you have to update your balance with that vendor, and your general ledger. You have to record the new inventory moving in. Perhaps the inventory has a barcode, or a serial number with a warrantee date, or a batch with an expiry date. The inventory movement will be tied to the cost of that invoice - remember to take into account those exchange rates. Also, there’s a variety of ways to account for item cost. You will have had one or more purchase orders - those have to be closed. If the vendor is to be paid in a different currency, you have to do every accounting entry in your currency and theirs, and record the exchange rate differences. Your accounts payable are now in that other currency, which means that has to be updated all the time for accurate bookkeeping. You have to note which employee was the buyer. This buyer has targets and metrics to be tracked. Perhaps there are reports and alerts set up for them. You will have a budget, which may impact this transaction. Of course you also have master data for the vendor, the employee, and the items bought.
There is a lot more to this than the above, and this is for the simplest business case - one that happens hundreds or thousands of times a day depending on the business. Usually there are special considerations to program into the above as well. Now imagine what happens when a complicated situation arises, where you have complex transactions spanning months, many countries, and many other businesses. Sometimes there are legal rules built into the logic. Usually those rules only alply to one country. And I haven’t even mentioned freight or tax or landed costs from secondary invoices.
SAP is a ball of wire because it simply has to be.
If you built a new ERP, you will end up the same.