Thats cause mappers are too damn lazy to make a .res file.
Here is what you do. Open up the .zip or .rar that came with the map and look at the files. Then, while that window is open, start a new window. Go to the maps folder. Start up notepad and put the location to all the files in the compressed folder. The files are relitive to the esf folder's location.
Like c:\steam\steamapps\email\half-life\esf\guru.wad becomes guru.wad in that folder. And c:\steam\steamapps\email\half-life\esf\sprites\l33tsprite.spr becomes sprites\l33tsprite.spr inside the notepad. BE SHURE TO LINK ALL FILES EXEPT THE MAP.
*VERRY IMPORTANT*
SAVE THE FILE AS THE FOLLOWING: mapname.res AND PUUT IT IN THE MAP FOLDER. So a map nammed esf_l33tmap_withsprite.bsp would have a .res file to match it called esf_l33tmap_withsprite.res(This is verry important) (verry important thing over)
Now take that .res file that you created, *VERRY IMPORTANT* ALONG WITH YOUR MAP AND ALL THE PROPER FILES IN THE PROPER PLACES (verry important thing over). And put them all on your server.
MESSAGE TO MAPPERS:
Make a damn .res file for each of your maps. AND ALSO INCLUDE SPAWN POINTS FOR A FFA GAME. So that way it reduces the trouble that us server people have to go through. Like useing entmod to create spawns and also havening to go through so much trouble makeing shure everything will be uploaded to a person's machine. (sorry Rock Lee, I dont mean to inslut you, but) Think of all the noob admins like above here. And all of those people that just create a listen server to think they can even handle the stress on thier connection.
~Thank you