They did. All of Iberia and Septimania (later Occitania) up to the mouth of the Rhone (Nimes) in the 720s. Then they mended the river in the campaign that ended in Tours (732).
What fucks up the Muslims and prevents them from holding the northern mountains for long is not military defeats like Tours, or Toulouse (721), or Covadonga (722); those were comparatively minor setbacks from which they could have recovered and come back with a bigger army if they wanted to. It's actually the Berber revolts in Africa and Spain from 739 onwards, which in Spain translate into all out ethnic war between Berber and Syrian garrisons. These surprise the Caliphate with its pants down in a moment when the Franks are striking back from Tours and invade Septimania; even so, the region was populated and well fortified, and had a long history of fighting off the Franks. The fact that it took the Muslims 5 years to submit the Septimania and 20 to the Franks (reaching the Roussillon in 760 only) speaks volumes about who they were more willing to live under, the Franks having to promise later to uphold Gothic laws in Septimania in order to get the Gothic nobles to defect.
Meanwhile in Spain, we have Asturias strenghtening itself and marrying into the Dukes of Cantabria, then taking Galicia (bigger than Asturias and Cantabria together) without a fight in 740 because the Berber garrison there just plain rebels and leaves. Then in 756 Abd-al-Rahman I arrives and wages his own war to take control of what's left of Al-Andalus for decades. He's not yet finished when the Abbasids attempt a landing in Murcia in 777, and Charlemagne marches on Zaragoza in 778, so Abd-al-Rahman, having bigger fishes to fry, is happy to secure peace with the by then well established kingdom of Asturias and leave it alone while he campaigns in the Ebro basin until 781. All through this time and up to his death Abd-al-Rahman had to fight off revolts and twart several assassination attempts too.
But what if the Berber revolt had been prevented? Then you give the Muslims a chance to fight back in the 740s-50s and prevent the establishment of the Ummayads in Cordoba, avoiding the additional decades of warfare that entailed...