SC-FDMA vs single carrier signal PAPR
Consider some actual use cases for further insight:

Unmodulated Carrier: The peak to average ratio is 3 dB given the relationship between the rms and peak level of a sine wave.

Unfiltered Single-Carrier BPSK or QPSK: This would be the modulation of rectangular shaped symbols, so would have a 3 dB peak to average ratio just as in the case of an unmodulated carrier. Great for peak-average ratio but terrible for bandwidth constraint. Given we have to share bandwidth efficiently (in terrestrial communications) we don't have this luxury. GPS is an example where the signal can be essentially unfiltered and we have the benefit of power efficiency.

Root-Raised Cosine BPSK or QPSK: This is what is traditionally done  but the tighter the filtering (smaller $\alpha$ in the filter design) results in more overshoot of the symbol transitions and slower trajectories that may go through the origin resulting in a higher peak-average ratios.

pi/4 QPSK: This modulation changes phase by $\pi/4$ between symbols thus avoiding having the slower transitions from filtering from ever going through the origin (Meaning minimizing the lowest instantaneous power in the signal envelope, increasing the average relative to the peak). This modulation is therefore considered for lower power (battery operated) applications since we are able to drive the transmit power amplifier further into saturation meaning higher efficiency.  

GMSK: Gaussian Minimum Shift Keying and other constant-envelope modulations have a 0 dB peak-average ratio and offer moderate bandwidth efficiency so are an excellent choice for power efficient (battery operated) applications. 

OFDM: To the extent each bin in the FFT representation of an OFDM symbol is independent and in consideration of each other randomly generated, the resulting signal as the summation of multiple independent identically distributed random processes will be well approximated by a Gaussian distribution given the central limit theorem. A Gaussian distribution has a relatively high peak to average ratio, and we need to decide on a significance to establish what is the "peak" (usually on a distortion criteria from clipping anything beyond the established peak). In SC-OFDM we do have some control over the association of the signals in each bin, allowing for lower peak-average ratios than MC-OFDM where each bin can represent a different data source.