That's not necessarily the case, it is updated at least every 3 months (and sometimes/often sooner, after 1-2 bills) and on each reassessment your available lines could increase by 1, or it could be as many as 2, 3 or 4 up to a usual max total of 5.
Your initial available lines would have been determined by your credit check on applying, but anything after this point is approved on an internal set of criteria by EE based on billing history, with possibly some remaining consideration of your initial credit check. If it gets to a point where you still don't have the option, you can contact EE and enquire whether they can do a credit referral to request a review for additional lines to be made available.
It is a thing because there unfortunately some people in this world who would gladly go to EE (or any other network) and take out contracts for 5 phones on day one with no intention of ever paying a bill, and those who may take on financial obligations which are beyond their means.
