Why was the traditional character 內 simplified to 内?
They don't call this a &quot;Simplification&quot; in the PRC. Simplification is stroke-cutting achieved by various methods - if a character is standardised not by stroke cutting, it's not a Simplification.
Basically, the structure of「內」was already not clear by the time of Han Dynasty clerical script, and famous calligraphers overwhelmingly wrote the inside with the same strokes as「人」. Most print forms and standardisation efforts (apart from the PRC) have reverted the inside back to「入」.

「內」(Baxter-Sagart OC: /*nˤ[u]p-s/, inside) is comprised of simultaneously semantic and phonetic「入」(/*n[u]p/, to enter) and semantic「宀」(building).
For reference, some character evolution sequences:
商甲鐵・185.1合集17299西周金大盂鼎集成2837秦簡日甲146背睡虎地秦簡東漢隸論語熹平石經殘今楷　
商甲存・812合集14993西周金公貿鼎・安集成2719秦簡編20・安睡虎地秦簡東漢隸校官碑・寶　今楷　
商甲前・4.28.3合集5560西周金豆閉簋集成4276秦簡秦・80睡虎地秦簡東漢隸衡方碑　今楷　

Please carefully note the shapes of the corresponding timeframes of the character samples.「人」was never part of the structure of「內」.
商甲甲・2940合集18901西周金善鼎集成2820秦簡日乙17睡虎地秦簡東漢隸孔龢碑　今楷　

What you're really seeing in the PRC standard「内」is the culmination of a change within regular script strokes using a Qin-Han shape as a basis.
Note in 東漢・衡方碑「內」, the top of the inner shape is still much more elongated than the top of 東漢・孔龢碑「人」. When regular script morphed characters into a roughly square shape (instead of the flattened rectangular shape as typical of clerical script characters), the top of「人」became elongated as well, so the inside of the regular script calligraphy style of「内」and「人」became indistinguishable.
隋?・智永楷眞草千字文　

References:

何琳儀《戰國古文字典：戰國文字聲系》
小學堂
中華語文知識庫・漢字源流
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》
孫雋《中國書法大字典》