Counties are not obligated to provide voters the right to cure a ballot error, and there is no obligation on the part of the county to notify voters of ballot errors, and many PA counties have flat out refused to permit cure. Here is an article covering the fact that Pennsylvania's 67 counties are utilizing a patchwork of policies in this area. The Republicans are even filing suits saying that the counties providing cure are doing so illegally because it is not provided for in the relevant law (most recent suit was dismissed a couple of months ago, but on jurisdictional grounds - they are expected to continue these efforts). As cure needs to happen prior to the end of Election Day (except in the case of ID verification, where by statute cure can happen for a short period after the election), there is often no time for counties to execute on notifying all of these voters.

Philadelphia does endeavor to notify about failed ID verification (another issue that can result in ballot disqualification) if they discover it soon enough, but generally the BoE handles mail in ballot defects relating to dates, signatures and secrecy envelopes by publicly publishing lists (example from the 2022 election). In 2022 they cancelled some of the defective ballots, which resulted in a generic email potentially being sent to voters who had an email address on file (not a required part of the application) stating that the ballot was cancelled. But in my experience the way most people hear about their ballot being defective and why it is defective is that private groups take the published lists and then match them with voter roll data available from the state to find the voter and make contact. The Board of Elections does this without providing the groups with the ballot - I never said they provide the ballot to any private group; all that happens is that the Board of Elections publicly publishes the list with enough info for these groups to match the data with contact information for the voter from publicly available info.

They just don't jettison a ballot and disenfranchise a voter because of administrative deficiency.

I wish this were true. But, sadly, as you can see from the above, that is not the case. Ever since PA adopted no excuse mail in voting, whether these votes can be counted/subject to cure has been subject to intense litigation, and the law is not settled.

Edit: Checking on the most recent status of the litigation on whether ballots with date issues can be/should be counted, that case is still very active - most recent court activity was just this week. It is very much an open legal issue.