Generally you should extract the relevant segment of the code into the question. It is inconsiderate to ask people to locate the 5 relevant lines in your 76 line program.
Your test is performing concurrent find-and-modify operations. Let's suppose there are two concurrent processes A and B that each increment account balance by 50. Starting balance is 0. The order of operations could be:
A: what is the current balance for account 1234?
B: what is the current balance for account 1234?
DB -> A: balance for account 1234 is 0
DB -> B: balance for account 1234 is 0
A: new balance is 0+50 = 50
A: set balance for account 1234 to 50
DB -> A: ok, new balance for account 1234 is 50
B: new balance is 0+50 = 50
B: set balance for account 1234 to 50
DB -> B: ok, new balance for account 1234 is 50
From the database's perspective, there are no "write conflicts" here. You asked to set the balance to 50 for the given account twice.
There are different ways of solving this issue. One is to use conditional updates such that the process looks like this:
A: what is the current balance for account 1234?
B: what is the current balance for account 1234?
DB -> A: balance for account 1234 is 0
DB -> B: balance for account 1234 is 0
A: new balance is 0+50 = 50
A: if balance in account 1234 is 0, set balance to 50
DB -> A: ok, new balance for account 1234 is 50
B: new balance is 0+50 = 50
B: if balance in account 1234 is 0, set balance to 50
DB -> B: balance is not 0, no update was performed
B: err, let's start over
B: what is the current balance for account 1234?
DB -> B: balance for account 1234 is 50
B: new balance is 50+50 = 100
B: if balance in account 1234 is 50, set balance to 100
DB -> B: ok, new balance for account 1234 is 100
As you see, the database must support the conditional update and the application must handle the possibility of concurrent updates and retry the operation.
If the balance can go up and down, this is not a practically useful way of writing a debit & credit system (but if balance can only increase or only decrease, this would in fact work quite fine). In real systems you'd use a special field whose purpose is to identify the specific version of the document that was in existence at the moment the application retrieved some data; the update is conditioned on the current version of the document staying the same, and each update increments the version. Concurrent updates would then be detected because the version number is wrong rather than a content field.
There are ways to produce a "write conflict" on the database side, for example by using transactions as supported by MongoDB 4.0+. In principle this works the same way but the "version" is called a "transaction identifier" and it's stored in a different place (not inline in the document being operated on). But the principle is the same. In this case the database would inform you that there was a write conflict, you'd still need to reissue the operations.
Update:
I think you also need to distinguish between "optimistic currency control" as a concept, its implementation, and what the implementation applies to. https://docs.mongodb.com/manual/faq/concurrency/#how-granular-are-locks-in-mongodb for example says:
For most read and write operations, WiredTiger uses optimistic concurrency control. WiredTiger uses only intent locks at the global, database and collection levels. When the storage engine detects conflicts between two operations, one will incur a write conflict causing MongoDB to transparently retry that operation.
Reading this statement carefully, it applies to write operations on storage engine level. I imagine when MongoDB performs something like $set, or other atomic write operations, this would apply. But this doesn't apply to application-level operation sequences like you've given in your example.
If you try your example code with your favorite relational DBMS, I think you'll find it produces roughly the same result as you've seen with MongoDB, if you issue a transaction around each individual read and write (such that balance read and write are in different transactions), for the same reason - RDBMSes lock data (or use techniques like MVCC) for the lifetime of a transaction, but not across transactions.
Similarly if you put both balance read and balance write on the same account into a transaction in MongoDB, you may find that you are receiving transient errors when other transactions modify the account in question concurrently.
Lastly, the API that MongoDB implements for transactions (with retries) is described here. If you look at it carefully you'll find that it expects the application to reissue not just the transaction commit command, but to repeat the entire transaction operation. This is because generally, if there is a "write conflict" the starting data has changed, and simply attempting the final write again isn't enough - potentially calculations in the applications need to be redone, possibly even side effects of that process change as a result.
