Inflation and p/e

Here's a random shower thought that I'm struggling with due to being up all night with my infant. Inflation erodes the value of earnings. So why then are we still held to historical standards of valuation? Should a 15 p/e in 1970 equal a 15 p/e now? Let's work it out. We'll pretend that company XYZ's market cap has remained flat for 50 years (at $5B) and their earnings have also remained flat (at 15x earnings). Their earnings have become far less valuable than they were 50 years ago. Why haven't valuation standards adjusted for inflation? $Nasdaq Composite Index (.IXIC.US)$ $Dow Jones Industrial Average (.DJI.US)$ $S&P 500 Index (.SPX.US)$ $Invesco QQQ Trust (QQQ.US)$ $SPDR S&P 500 ETF (SPY.US)$