What’s up with that?
If you’re wondering why you personally can’t, it’s because it doesn’t exist. If you’re suggesting that we should be able to have more than 1 advanced house, then you should have posted in the idea category.
As @Ruskin505 have said, the main reason is that it is not yet available. Maybe in future updates.
Because that would allow you to accumulate free recruits faster, which results in more training camp entries, then more free feeders and TC20 summons, and SG couldn’t have you enjoying all that.
(Only slightly being sarcastic)
for high level players there is no need for more recruits whatsoever, so I am not sure why this would be required. I often leave my recruits full and farm away to enable me to store up more food.
Until a training/hero academy option requires more than the max (somewhere around 320ish I believe) they will keep it as is. HA10 requires 300, so until SH gets another level and they introduce another advacned building or more levels to existing advanced buildings it will stay the same.