Or do these image creators not pay much attention to how Rāma really looked and missed this important characteristic of his?
Yes, you are right. Moreover, we don't know if Vālmīki is using superlative terms in his poetry and did not mean his readers to take all those physical descriptions literally.
In many other instances, he uses words such as dīrghabāhu and mahābāhu for Hanumān, Daśaratha, Lakṣmaṇa, Rāvaṇa, Indrajit and so on.
तत्राकरोन्महायुद्धं राजा दशरथ स्तदा |
असुरैश्च महाबाहुः शस्त्रैश्च शकलीकृतः || ९-२-१५
tadaa = then; dasharathaH raajaa = king Dasaratha; akarot = created; mahaayuddham = great battle; tatra = there; mahaabaahuH = the long-armed Dasaratha; shakaliiR^itaH cha = was made into pieces; shastraiaH = with weapons; asuraiH cha = by demons.
Then, king Dasaratha fought a great battle there. With their weapons, demons made into pieces the long-armed Dasaratha.
स तम् गृह्य महाबाहुः काल आयस परिष्कृतम् || ५-५३-४३
रक्षिणः तान् पुनः सर्वान् सूदयामास मारुतिः |
saH mahaabaahuH = that long-armed; maarutiH = Hanuma; punaH = again; gR^ihya = taking; tam = that rod; kaalaaasapariSkR^itam = made of iron; suudayaamaasa = killed; sarvaan = all; taan rakSiNaH = tose guards.
That long-armed Hanuma, again taking that rod made of iron, killed all those guards.
Both Rāma and Lakṣmaṇa are long-armed:
दीर्घ बाहू विशालाक्षौ शर चाप असि धारिणौ |
कस्य न स्यात् भयम् दृष्ट्वा हि एतौ सुर सुत उपमौ || ४-२-२०
diirgha baahuu vishaala akshau = lengthy arms, broad eyed ones; shara chaapa asi dhaariNau = arrow, bow, sword, wielders of; kasya na syaat bhayam = to whom, will not, be there, scare; dR^iSTvaa etau = on seeing, these two; sura suta upamau = Divinity's, sons, in simile.
"Lengthy are their arms, broad are their eyes and they are the wielders of arrows, bows and swords... and on seeing them, to whom there will be no scare? These two are like the sons of some divinity..." Sugreeva thus spoke to Hanuma. [4-2-20]
हन्तुकामस्य मे बाष्पं चक्षुश् चैव निरुध्यते |
तदेवैष महाबाहुर्लक्ष्मणः शमयिष्यति || ८९-६-१८
वानरा घ्नन्तुं सम्भूय भृत्यानस्य समीपगान् |
me = my; chakShuH = yes; nirudhyati = are clouded; baaShpam = with tears; hantukaamasya = even when I wish to kill him; eShaH = this; mahaabaahuH = long-armed; lakShmaNaH eva = Lakshmana alone; shamayiShyati = will finish; tam = him; vaanaraaH = O monkeys!; sambhuuya = all of you together; ghnata destroy; bhR^ityaan = his servents; samiipagaan = standing beside; asya = of him.
My eyes are getting clouded with tear, even when I think of killing him. This long-armed Lakshmana alone will finish him. O Monkeys! All of you, together, kill his servants standing beside him.
तम् उद्वीक्ष्य महाबाहुः पिन्ग अक्षम् पुरतः स्थितम् |
रोषेण महता आविष्टो रावणो लोक रावणः || ५-५०-१
स राजा रोष ताम्र अक्षः प्रहस्तम् मन्त्रि सत्तमम् |
saH raavaNaH = That Ravana; mahaabaahuH = the long armed; lokaraavaNaH = who causes the people to cry; udviikSya = looking up; tam piN^gaakSam = at that mokey; sthitam = standing; purataH = in front of him; mahataa kopena aaviSTah = affected with violent anger; shaN^kaahR^itmaa = with his mind seized with suspicion; dadhyau = speenlated; kapiindram = about Hanuma the formost of monkeys; aavR^itam = who was invested; tejasaa = with splendour.
That Ravana, the long-armed, who caused people to cry, looking up at that monkey standing in front of him, was affected with violent anger, having his mind seized with suspicion, speculated (as follows) about Hanuma, the jewel among the monkeys, who was invested with splendour.
Apart from few physical characteristics, Vālmīki also attributes 16 nonphysical qualities to Rāma. Imagine an artist trying to incorporate all these in his painting; I'm not sure if we will like the final image.