Why the standard errors obtained from PROC GLM (for analysis of multiple regression) is larger compared to those from PROC QUANTREG (for analysis of quantile regression)?
Simple answer: Since they are estimating different things, there is no reason to expect they would have the same standard errors. This is especially true for quantiles other than the median - which should at least be similar to the mean being estimated in OLS regression.
Standard errors for quantile regression are calculated in various ways. SAS PROC QUANTREG allows three methods and, by default, uses various methods (see the link).
To determine why the SE is smaller for quantile regression in your particular case, you would have to compare the formulas for your particular data. However, one general reason could be that quantile regression is less affected by outliers or other extreme points.