If you buy extract from your LHBS then it’s quite possible to weigh out whatever amount of LME you want. If you’re ordering online or using imported LME then you’re likely committed to the standard container sizes used by the online source or manufacturer. Extract recipes based on a specific supplier’s packaging tend to reflect this.
LME is less expensive, easy to use in large quantities, available in more varieties and brands, and stales faster. DME is more readily available in smaller quantities, stays fresher longer, stays fresher longer once the package is open, and (to me) is easier/more convenient to use and keep around once the package is opened. For some or all of these reasons recipes tend to specify multiples of standard package weights of LME for the bulk of the fermentables, then build up any remaining fermentables with DME.
For instance an English pale ale recipe will likely suggest an English brand extract like Munton’s, and an ESB may need between two and three 3.3 lb cans of Munton’s LME in a 5 gal batch to provide an OG appropriate for the style. So to achieve that gravity an ESB recipe may specify 6.6 lbs (two cans) of Munton’s LME plus some Munton’s DME to make up the difference. Same thing with online homebrew shops who build recipes/kits based on their LME packaging. For instance the Consecration clone from Morebeer comes with one of their standard sized pouches of LME and a baggie of DME to bump the OG up to where Vinnie says it should be.