That's not entirely true. Mindflayers aren't robots. I think people equate being souless to having no emotions, but they actually do feel emotions. It's just extremely unlikely and rare that they form relationships and feel happiness. Both the Emperor and Omeluum are already anomalies for mindflayers, so it's not super far out of the scope to see them experiencing something rare to others of their kind. I'm not sure if that's a direction Larian would want to take the Emperor, but it could theoretically happen.
To quote the wiki,
"To non-illithids, mind flayers seemed calm and collected at almost all times, dispassionate and seemingly emotionless outside of their constant desire to dominate others. They dismissed basal emotions like pride as fatal flaws and founded themselves on a pragmatic outlook. While they might occasionally seem furious, it was difficult to discern if this was actual feeling or a ruse to manipulate others. Theories on this ranged from the belief that the mind flayers had no emotions, very few, or extreme levels of self-control, but all of these were incorrect. In truth, illithids had a whole spectrum of emotions and felt them intensely, sometimes even behaving irrationally because of them, but these feelings were almost entirely internalized and not evident even during moments of raging inner turmoil.
However, while mind flayers did indeed have a range of emotions, these emotions were almost entirely negative. They felt anger and hate when foiled and stymied in their ascent to sovereignty, fear when faced with hostile minds they could not control, shame when incapable of controlling minds, envy towards vast knowledge that was not their own, abhorrence towards a wasteful use of thralls, disgust for those that would engage with lesser beings on an equal basis, and sadness when a compatriot died far from home. These emotions, and related feelings such as anxiety and contempt, made up most of an illithid's emotional repertoire.
The most commonly experienced illithid emotion was frustration, discontentment rooted in the fact that they had not yet achieved dominion, and this dissatisfaction was a subtle, undercurrent that constantly defined them and colored their other emotions and thoughts. The same principle of underlying negativity could be found in the illithids overall; mind flayers had no sense of true happiness, and so did not plan to become happy.
The closest most illithids came to being happy was the delight of consuming a brain, but even then the act had such sadistic overtones that it still wouldn't be "happiness" as normally defined. A mind flayer's highest emotional state was that of self-satisfaction, whether that be from a personal sense of pride or the satiation of their curiosity, and it was this that motivated their behavior. They sought to live in luxury, to feast on the minds of well-bred thralls and master their psionic birthright. Love, or even friendship outside of acquaintances, was almost unheard of, for they had supernatural means of filling these emotional voids."
and this quote here too,
"The illithids had to dominate not only to achieve their goals, but to fulfill certain basic needs of their own. Without a mind to control, a mind flayer would feel incomplete. They actually had an intimate relationship with their own thralls, suffering when they died (whether by sickness, age, or physical harm) and sometimes going mad from loneliness without their constant companionship. Illithids were known to postpone their other goals just to renew an emptied retinue of thralls and every illithid had at least one personal thrall. When they found one they favored, illithids would go out of their way not to eat personal thralls in bouts of hunger or anger, and might even grant them toys and trinkets to occupy themselves with when not working. "
