Why can't Unix programs have signals with meaningful program defined names (rather than USR1, etc)?

The signals available on an OS are defined by the OS (usually following POSIX) - they're not "strings" but rather integer constants with standard names. USR1 and USR2 are the two signals that have no attached specific meaning - intended for whatever arbitrary use the developer wants.

On your linux machine, read man 7 signal for an overview of signal handling and signals.

You can redefine the meaning of other signals if you're prepared to deal with the OS issuing those signals in response to events. You can e.g. make HUP mean "reload configuration" - as long as you're either certain that the process will never get a hangup (terminal loss), or you're prepared to handle cases where the OS and not a user sends the HUP signal.

HUP is short for "hang-up". This signal is sent to a process if its controlling terminal reaches end-of-file. In the old days, controlling terminals were typically attached to serial ports, possibly via a modem link over a telephone line. If the telephone connection was hung up, the local modem would lower the Carrier Detect line, which would result in the kernel reporting end-of-file and a SIGHUP signal being sent.

WINCH is short for "window change". It is sent to a process if its controlling terminal changes size. For obvious reasons, terminals that can change size are usually pseudo-terminals ultimately represented by a terminal emulator running in a windowing environment (like xterm).

Try kill -l and find the answer yourself:

1)  SIGHUP       2)  SIGINT       3)  SIGQUIT      4)  SIGILL       5)  SIGTRAP
6)  SIGABRT      7)  SIGBUS       8)  SIGFPE       9)  SIGKILL      10) SIGUSR1
11) SIGSEGV      12) SIGUSR2      13) SIGPIPE      14) SIGALRM      15) SIGTERM
16) SIGSTKFLT    17) SIGCHLD      18) SIGCONT      19) SIGSTOP      20) SIGTSTP
21) SIGTTIN      22) SIGTTOU      23) SIGURG       24) SIGXCPU      25) SIGXFSZ
26) SIGVTALRM    27) SIGPROF      28) SIGWINCH     29) SIGIO        30) SIGPWR
31) SIGSYS       34) SIGRTMIN     35) SIGRTMIN+1   36) SIGRTMIN+2   37) SIGRTMIN+3
38) SIGRTMIN+4   39) SIGRTMIN+5   40) SIGRTMIN+6   41) SIGRTMIN+7   42) SIGRTMIN+8
43) SIGRTMIN+9   44) SIGRTMIN+10  45) SIGRTMIN+11  46) SIGRTMIN+12  47) SIGRTMIN+13
48) SIGRTMIN+14  49) SIGRTMIN+15  50) SIGRTMAX-14  51) SIGRTMAX-13  52) SIGRTMAX-12
53) SIGRTMAX-11  54) SIGRTMAX-10  55) SIGRTMAX-9   56) SIGRTMAX-8   57) SIGRTMAX-7
58) SIGRTMAX-6   59) SIGRTMAX-5   60) SIGRTMAX-4   61) SIGRTMAX-3   62) SIGRTMAX-2
63) SIGRTMAX-1   64) SIGRTMAX