Many that would qualify as AP-bruiser scale well with AS, so they have a different stat to use, most AP-bruisers have good base rather than ratios and thus work better with pen and the amp from abyssal and liandrie's, and also there is one major group of stats that are important for bruiser and not so much for mages: DEF STATS.
The issue for AP bruiser is not that they lack 'other stats' that they need/want, the issue is that they still rely to much on AP to get proper values while haveing poor options for the other stats they need.
With the current PBE changes to ragebkade, Nashors is about the only remaining AS/AP item, and it sadly has alot power in the AP-ratio of the on-hit, so it also relys on getting more AP rather than building tanky.
Also, the AP-tank Options are not so great, Abyssal and RoA share a passive, Zhonyas is not that great for sustained combat patterns, There is no other AP-armor item, Banshies is also not the ideal thing for a beefy fighter, and asides from RoA, all the AP-hp items are pretty low key on the HP side.
Though the main issue is that both through Nashors and theit own kits, AP-bruisers rely to much on high-ish AP values to funktion so they can't really opt into pure tank items like most ADs can.
Also,
is kinda a must buy later in the game, they lack a more tanky alternative while ADs have BC and Conqueror to deal with high resis later on.
Though mana scaleings might work, and maybe giveing
adaptive damage, it would make
items more viable for them.