When an adjective comes before a noun with no article or possessive at all — common with mass nouns (Kaffee, Milch, Wasser), bare plurals, and after words like viele/einige/mehrere that don't decline like an article — there's nothing else in the phrase showing gender and case. So the adjective takes the "strong" ending: almost exactly what der/die/das/die themselves would have been.

Quick test: compare this table to the der-word (der/die/das/dem...) endings themselves — masculine nominative -er, neuter nominative/accusative -es, masculine/neuter dative -em, feminine dative -er, plural dative -en — the adjective is standing in for the missing article, so it copies its ending almost exactly.