Keras not training on entire dataset

The number 1875 shown during fitting the model is not the training samples; it is the number of batches.

model.fit includes an optional argument batch_size, which, according to the documentation:

If unspecified, batch_size will default to 32.

So, what happens here is - you fit with the default batch size of 32 (since you have not specified anything different), so the total number of batches for your data is

60000/32 = 1875