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That's just a convenient value that is not that difficult to achieve in modern IC op amps with a few stages and is high enough so that the input differential signal voltage to achieve the maximum output swing of the op amp is low enough to be negligible (close to the ideal of 0V) for most applications.
For example the differential input voltage to achieve 10V output with gain of 100,000 is only 100µV.
THe gain is big enough to assume many things. But, more important factor is the Gain-BandWidth product (GBW) in MHz as negative feedback trades off these two factors.
The range is is from 0.002 to 10,000 for GBW now and the biggest variables are power consumption and CMOS technology.
The internal compensation is set to make the Op Amp look like a 1st low pass filter so there is no instability problems with unity gain and full negative feedback.
For example if GBW was 1MHz it could mean the DC gain was 1 million with a breakpoint of 1 Hz or a gain of DC gain of 100k and breakpoint of 10 Hz It doesn't matter because open loop gain would never be used as a linear amp.
According to Black`s formula for an amplifier with negative feedback the closed-loop gain is
Acl=Ao/(1+Ao*Fr)=1/(Fr+1/Ao) with Ao=open-loop gain, Fr=Feedback factor.
As you can see, for Ao>>1/Fr the closed-loop gain is less dependent on Ao.
For very large Ao values (1E5..1E6) the closed-loop gain Acl practically is determined by the feedback factor Fr only.
This a very convenient because (a) tolerances of Ao do not enter the value of Acl and (b) the transfer function (amplifier, integrator, filter) is determined by the external components only.
This property gives the opamp his name: Operational amplifier (to realize different mathematical operations)
Also, open loop gain is seldom really that well specified, because it usually does not much matter (As long as it is large), and because it is somewhat sensitive to process variations.
Note also that this is a DC gain, by the time you are at even a few hundred Hz it is falling.
For 90+ percent of the use cases, the gain of the circuit is defined by the external feedback components with more then sufficient accuracy, it is somewhat rare for open loop gain to be a issue (GBP on the other hand can be, as can things like offsets and bias current).