Why I can not use Map in dynamic query

Use an extra set variable

set<Id> setAccountIds = new set<Id>();

Map<Id, Account> maps = new Map<Id, Account>([Select Id from Account]);
setAccountIds.addAll(maps.keySet());
String s = 'Select Id from Account Where Id IN :setAccountIds ';

List<Account> acc =   Database.query(s);
System.debug('@@' + acc);

from the Apex Developer guide

"You can use simple bind variables in dynamic SOQL query strings:

for more info about the binding

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_dynamic_soql.htm

Yes, We have to store map.KeySet or map.values() in either list/set and then use that set and list over there. Because , we can use variable and collection in dynamic SOQL and map.keySet() / map.values() are the method of collection type(map).