Why Have Modern *duilian*'s become 成语+3 Random Characters?
This observation is merely coincidental. The first 4 characters need not come from classical Chinese idiom (成語), and the remaining 3 are by no means random.
Couplet (Duilian) has specific rules. The linked article has a list of requirements and its etymology.
For example, the cliché duilian

天增歲月人添壽
春滿乾坤福滿門

&quot;天增歲月&quot; is not a recognized chengyu, but means &quot;As heaven attains greater age&quot;; &quot;人添壽&quot; is not random but has to relate to it, meaning &quot;people enjoy longer years&quot;.
Moreover, a line of 7 characters is common but it is not a requirement.
For example, the famous (but modern (19th century)) couplet in Temple of Zhuge Liang in Chengdu

能攻心則反側自消從古知兵非好戰
不審勢即寬嚴皆誤後來治蜀要深思

also meet the requirement.